How to Generate Json Post Request Body? 400 Bad Request

I am writing API Test scripts for an application for work and I need help on figuring out what goes in to the request body for Posts and Put requests (using raw-JSON).

I have access to the Java classes that have the @Columns defined. I assumed you took those and put them in the request body like so:

{
“id”: “{{id}}”
“createdBy”: “{{createdBy}}”
etc.
}

However, I am always getting 400 Bad Request. I am new to Postman so I am not sure what exactly goes into the request body or what exactly the API is supposed to return. Anyone have any tips?