API is function binded to particular web uri, and every function defines it arguments type and place to accept input ( body or header param or path param.. etc)
So you just cannot pass parameters in request body as json, if the calling API is expected to take input as query params.
How to send parameters to any API (as body or as query params) are defined by API developer, If it is publicly exposed API, then there will be documentation defined API request and response.
Edited ( to add more information based on comment)
Behaviour of http methods are defined in RFC documents and these documents are updated time to time with new features or bug fixes.
The API can be written by developer using older version of HTTP server or client, that doesn't allow certain features.
Whereas postman as a tool wants to keep it ahead of race, so it has added new features so everyone who is using either old or new can use.