Has anyone worked with Multi-part/mixed Request Body as it seems it can't able to read variables

My question: I’m trying to create a dynamic request body where in I’ll be pulling most of the Key Values from Variables, but it seems it is not supported yet? Maybe anyone can share a solution related to the issue I’m facing.

Details (like screenshots):

I’ve already tried: I’ve tried assigning the json part of the request body as variable but it is not working as well.