HTTP requests (as REST-Calls are), are just plain text in the end of the day. They do not care if you send your data as JSON, XML or in the form-data definition.
Here is an example of how a POST request is sent when form-data is being used as input - (full documentation):
I cannot test it right now, but it appears to me that you need to concenate your form data fields in a simple, one-line text string. In this case, your request body would just be say=Hi&to=Mom.
No idea if we´d have to change the content-type or something else - maybe try it out and see if it works 🙂
