How to script changes to message body content for Rest POST request ?

Is there a way via scripting to change the content of the message body for a Rest POST request ?

As opposed to hard coding the content ?

I see it is possible to append a file to a POST request, is that the only way ?  I am not sure if that will achieve desired result.

I saw something about importing xml schema, but not sure how it works, or if it does what I need.   Would like the ability to put json into message body as opposed to xml.

Thanks.