OpenIntegrations/docs/en/examples/HTTPClient/StartMultipartBody.txt

    URL = FunctionParameters["HTTP_URL"];
    URL = URL + "/post";

    Image = FunctionParameters["Picture"]; // URL, Path or Binary Data

    Result = OPI_HTTPRequests.NewRequest()
        .Initialize(URL)
        .StartMultipartBody() // <---
        .AddMultipartFormDataFile("file1", "pic.png", Image, "image/png")
        .AddMultipartFormDataField("Field1", "Text")
        .AddMultipartFormDataField("Field2", "10")
        .ProcessRequest("POST")
        .ReturnResponseAsJSONObject();