Is it me or is the App Connect API form missing being able to set the body for a POST JSON form?

I could be missing some coffees today but I can’t find for the love of god how to send a json in the body of the post for an API form.

In other words. How do I set the payload for an API form POST?

There is a difference in behaviour between the API action and the API form.

The API action can set the payload via dmx-data:q="'value'"

But this is not implemented in the API form, missing from the UI and/or not documented.