firebase - Firestore REST API starting query

Hello I am new to firestore and I'm trying to use the rest API to query from a collection called Users, find all users who have a field called about with the value test2

This is my POST request :

https://firestore.googleapis.com/v1beta1/projects/{myprojectid}/databases/(default)/documents/Users:runQuery

Body:

{"structuredQuery": {"where" : {"fieldFilter" : { "field": {"fieldPath": "about"}, "op":"EQUAL", "value": {"string": "test2"}        }    },"from": [{"collectionId": "Users"}]    }}

I get a response

{"error": {"code": 400,"message": "Invalid JSON payload received. Unknown name \"structured_query\" at 'document': Cannot find field.","status": "INVALID_ARGUMENT","details": [            {"@type": "type.googleapis.com/google.rpc.BadRequest","fieldViolations": [                    {"field": "document","description": "Invalid JSON payload received. Unknown name \"structured_query\" at 'document': Cannot find field."                    }                ]            }        ]    }}

Can someone tell me what I am doing wrong ? Thanks a lot. I'm stuck and unable to proceed.