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

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

    Text     = "Hello world!";
    Encoding = "Windows-1251";

    Result = OPI_HTTPRequests.NewRequest()
        .Initialize(URL)
        .UseEncoding(Encoding) // <---
        .SetStringBody(Text)
        .ProcessRequest("POST")
        .ReturnResponseAsJSONObject();