Trying to create googleSheets from REST api

Hi Postmen
I’m trying to create a googleSheets on a service account.
using the Http code
but I can’t find how to specify my key.json that was created by the google developer console.
what is the syntax of the HTTP request
(note i need to use http, not python or anything else because afterwards i need to get this code to work from a proprietory 4GL

thanks !
also note: i read on several forums that i have to grant this googlesheet to my service account. I think I cant do this because these spreadsheets will be created on the fly when a service call is create. i.e. heaps of them.