// Configuration
$myConfiguration = [
    "Account" => "CH4431999123000889012",
    "CreditorName" => "Muster AG",
    "CreditorAddress1" => "Hauptstrasse 1",
    "CreditorAddress2" => "8000 Zürich",
    "CreditorCountryCode" => "CH",
    "DebtorName" => "LivingTech GmbH",
    "DebtorAddress1" => "Dörflistrasse 10",
    "DebtorAddress2" => "8057 Zürich",
    "DebtorCountryCode" => "CH",
    "Amount" => "1.50",
    "ReferenceNr" => "21000000000313947143000901",
    "UnstructuredMessage" => "Mitteilung zur Rechnung",
    "Currency" => "CHF",
    "QrOnly" => "false",
    "Format" => "PDF",
    "Language" => "DE",
];

// Call function to create invoice
$myFile = generateQrInvoice($myConfiguration);

// Work with binary data
if($myFile != null) {
    // ...
}

function generateQrInvoice($myRequestConfiguration) {
    // Main configuration
    $myEndpointUrl = "http://v2.qrbillservice.livingtech.ch";
    $myEndpointPath = "/api/qrinvoice/create/";
    $myApiKey = "mySecretApiKey";

    // GET parameters
    $myGetParams = http_build_query($myRequestConfiguration);

    // CURL
    $myCurl = curl_init();
    curl_setopt($myCurl, CURLOPT_URL, $myEndpointUrl . $myEndpointPath . "?" . $myGetParams);
    curl_setopt($myCurl, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($myCurl, CURLOPT_HTTPHEADER, array(
        "APIKEY: " . $myApiKey, 
        "Accept: application/json"
    ));

    // Debug only
    curl_setopt($myCurl, CURLOPT_SSL_VERIFYHOST, false);
    curl_setopt($myCurl, CURLOPT_SSL_VERIFYPEER, false);

    try {
        // Perform request
        $myResponse = curl_exec($myCurl);

        // Check status
        if (!curl_errno($myCurl)) {
            if(curl_getinfo($myCurl, CURLINFO_HTTP_CODE) == 200) {
                // Read and parse JSON
                $myJsonObject  = json_decode($myResponse, true);

                // Check if error
                if($myJsonObject['isSuccess'] == "true") {
                    if(isset($myJsonObject['base64Content']) && !empty($myJsonObject['base64Content'])) {
                        // E.g. save file
                        file_put_contents($_SERVER['DOCUMENT_ROOT'] . "/output/" . uniqid("", true) . ".pdf", base64_decode($myJsonObject['base64Content']));

                        // Return data
                        return base64_decode($myJsonObject['base64Content']);
                    } else {
                        throw new Exception("no data provided");
                    }
                } else {
                    throw new Exception($myJsonObject["message"]);
                }
            } else {
                throw new Exception("status code " . curl_getinfo($myCurl, CURLINFO_HTTP_CODE));
            }
        } else {
            throw new Exception(curl_error($myCurl));
        }

        // Close curl
        curl_close($myCurl);
    } catch(Exception $e) {
        // Handle exception
        echo "Error: " . $e->getMessage();
        return null;
    }
}

// Configuration
let myConfiguration  = { 
    "Account" : "CH4431999123000889012",
    "CreditorName" : "Muster AG",
    "CreditorAddress1" : "Hauptstrasse 1",
    "CreditorAddress2" : "8000 Zürich",
    "CreditorCountryCode" : "CH",
    "DebtorName" : "LivingTech GmbH",
    "DebtorAddress1" : "Dörflistrasse 10",
    "DebtorAddress2" : "8057 Zürich",
    "DebtorCountryCode" : "CH",
    "Amount" : "1.50",
    "ReferenceNr" : "21000000000313947143000901",
    "UnstructuredMessage" : "Mitteilung zur Rechnung",
    "Currency" : "CHF",
    "QrOnly" : "false",
    "Format" : "PDF",
    "Language" : "DE"
}

// Call function to create invoice
let myFile = generateQrInvoice(myConfiguration);

// Work with binary data
if(myFile != null) {
    // ...
}

function generateQrInvoice(myRequestConfiguration) {
    // Main configuration
    let myEndpointUrl = "http://v2.qrbillservice.livingtech.ch";
    let myEndpointPath = "/api/qrinvoice/create/";
    let myApiKey = "mySecretApiKey";

    // GET parameters
    let myGetParams = new URLSearchParams(myRequestConfiguration);

    // Perform request
    fetch(myEndpointUrl + myEndpointPath + "?" + myGetParams, {
        method: "GET", 
        mode: "cors", 
        cache: "no-cache", 
        headers: {
            "APIKEY": myApiKey, 
            "Accept": "application/json"
        }
    }).then(function (myResponse) {
        try {
            // Check status
            if(myResponse.status == 200) {
                // Read and parse JSON
                let myJsonObject  = JSON.parse(myResponse);

                // Check if error
                if(myJsonObject["isSuccess"] == "true") {
                    if("base64Content" in myJsonObject && myJsonObject["base64Content"].trim() != "") {
                        // E.g. send file to client
                        let myBlob = new Blob(atob(myJsonObject["base64Content"]), {type: "application/pdf"});
                        let myBlobUrl = URL.createObjectURL(myBlob);
                        window.open(myBlobUrl);

                        // Return data
                        return atob(myJsonObject["base64Content"]);
                    } else {
                        throw "no data provided";
                    }
                } else {
                    throw myJsonObject["message"];
                }
            } else {
                throw "status code " . myResponse.status;
            }
        }
        catch(e) {
            // Handle exception
            console.warn("Error: " + e.message, e);
            return null;
        }
    }).catch(function (err) {
        // Handle exception
        console.warn("Error: " + err.message, err);
        return null;
    });
}

[...]
    'Create Request Object
    Set objRequest = CreateObject("MSXML2.XMLHTTP")

    'Run request
    With objRequest
        .Open "GET", strUrl, blnAsync
        .SetRequestHeader "Accept", "application/json"
        .SetRequestHeader "APIKEY", Trim(Range("B23").Value)
        .Send
        
        'Wait for response
        While objRequest.readyState <> 4
            DoEvents
        Wend
        
        'Process response
        strResponse = .ResponseText
    End With

    'Parse JSON-response
    Set dic = ParseJSON(strResponse)
    
    'Check for error
    If LCase(Trim(dic("obj.isSuccess"))) = "true" And Len(Trim(dic("obj.base64Content"))) > 0 Then
        'Use workbook path as tempory path
        strTempPath = Application.ActiveWorkbook.Path & "\temp.png"
    
        'Save byte array to tempory file
        Open strTempPath For Binary As #1
           Put #1, 1, DecodeBase64(dic("obj.base64Content"))
        Close #1
    
        'Insert image from tempory file
        Set Image = ActiveSheet.Pictures.Insert(strTempPath)
        Image.Top = Range("D1").Top
        Image.Left = Range("D1").Left
        Image.ShapeRange.Height = 200
        Image.ShapeRange.Width = 200
    
        'Delete tempory file
        Kill strTempPath
    Else
        'Error
        MsgBox ("Es gab einen Fehler")
    End If
[...]

Hinweis: der oben aufgeführte Code stellt nur einen Ausschnitt dar. Bei Bedarf kann eine Beispiel-Implementierung der API für MS Office/VBA heruntergeladen werden (für API V.1 / für API V.2). Der Code kümmert sich um die Basics wie:

• Validierung der Daten
• Aufbau und Durchführung des API-Requests
• Parsen der JSON-Daten
• Umwandlung von Base64-Content in PDF/PNG
• Einfügen der Grafik ins Office-Dokument

Sämtliche individuellen Arbeiten können auf Basis des Beispiels mit den nötigen VBA-Kenntnissen problemlos umgesetzt werden. Bei Fragen stehen wir Ihnen gerne zur Verfügung.