SugarCRM REST API - Filter POST - Related Module Fields

I'm trying to retrieve, using REST API, a list of records and one of its related module fields. Let's assume Accounts and Opportunities.

So, API Documentation (in the GET Filter) talks about defining related module in the fields parameter:

According to the same documentation, it would provide a result similiar to:

That's exactly what I need, but I'm trying to achieve this using the POST method. So, following the same path, I'm sending (using Postman):

PS: I tried all combinations of double quotes. Escaped, not escaped, with or without it, all of them give me the same result, that is:

The message is in pt-BR but it means "One of your request parameters is wrong". The HTTP status code is 422 - Unprocessable Entity.

What am I doing wrong? I tried everything and just don't know how to make it work. Looks like the documentation talks about something that simply don't work or doesn't exist.