REST Get and REST Post are 2 distinctive snap.
As for providing JSON body in a REST POST. It is quite straight forward with this POST to Slack as an example:
Plug in your Slack channel REST URL. “HTTP entity” is essentially your BODY and $ represent the JSON document you are getting from the input. If you have multiple docs feeding into your input, it will execute as many REST POST per doc. If you like to only execute once then you could use Group by snap to push an array into 1 document.
Note: you also have an option to place the actual JSON body in the “HTTP entity” field as well.
For Slack it only needs “text” in the body of REST POST, so I can feed the message I want to it.