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

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

    Result = OPI_HTTPRequests.NewRequest()
        .Initialize()
        .SetURL(URL)
        .AddHeader("X-Header1", "Value1") // <---
        .AddHeader("X-Header2", "Value2") // <---
        .ProcessRequest("GET")
        .ReturnResponseAsJSONObject();