There is none. "sending a URL request" is inherently not the same as POSTing something, which is what's needed as per the REST specs.
Why do you want to do it this way? Better yet, what is your end goal?
I am attempting to have the Pebble watch send a simple on request to a photon when the magnetometer is on a specific heading. I was hoping to just send a simple URL string but now with your information I might automate the webpage to post the form after it has been loaded by the URL request from the watch. I think