This might work using the built in HTTP Request functionality. The main concern is that the body XML string needs to be url encoded for this to work properly. Does this happen automatically or does this need to be done with another call or manually?
I don't know if this functionality is available to all versions or what version you may be running.
C_TEXT($response)
// build the form inside the body, e.g. `field1=value1&field2=value2`
$body_t:="query=...rest of xml goes here"