POST request to add and remove a user to a "Team Site Owners" group in SPO

I am trying to send a POST request to add a user to a "Team Site Owners" group in SPO via REST API calls. I am able to make other REST based calls (e.g. to get list items, update list items etc.). But I not able to figure what will be below fields for adding/removing a user from a Site Group:

1. REQUEST URL
2. REQUEST BODY

I am NOT using any JavaScript due to certain limitations and have to stick with normal REST based calls.

Any pointers or samples for this will be highly appreciated.