You must use the HttpClient object in the following way

HttpClient inv_httpClient
INTEGER li_return, li_status_code
String ls_url, ls_json, ls_returndata

ls_url = "http://12.34.56.78/blabla.php"
ls_json = '{"user":"blabla", "company":"blabla3", "branch":"blabla4" }'

inv_HTTPClient = CREATE HTTPClient

inv_HttpClient.SetRequestHeader("Content-Type", "application/json;charset=UTF-8")

li_return = inv_HttpClient.SendRequest("POST", ls_url, ls_json)

li_status_code = inv_HttpClient.GetResponseStatusCode()

IF ai_return < 0 THEN
// ERROR
END IF
IF li_status_code <> 200 THEN
//If the Status Code from the Web Service is different from "OK"
//Notify the user of the Error Message
MessageBox("Send Request", String(li_status_code) + ": "+ String(ahc_httpclient.GetResponseStatusText()))
END IF

li_return = inv_HttpClient.GetResponseBody(ls_returndata, EncodingUTF8!)

IF li_return = -1 THEN
//Notify the User of the Error
MessageBox("Post JSON", "Get response body failed.")
//Return Failure!
RETURN
END IF

RETURN ls_returndata