I want to make a request to this url https://api.uber.com/v1.2/requests/{request_id}/receipt, but I don't know where I get this request_id. Somebody knows?
You would get the request ID from posting a request. More information can be found on making a request here: https://developer.uber.com/docs/riders/references/api/v1.2/requests-post
So the trip flow goes: your app makes a request on behalf of a user, you receive a request ID. After the trip is completed you would the call that URL you specified to provide/get a receipt for that trip.