post https://gateway.stage.bill.com/connect/v3/login
Sign in to your BILL developer account. In the response, your API session is created and a BILL-generated sessionId is available. Use the sessionId in all subsequent API calls to confirm that you are in a signed-in session.
You can sign out with POST /v3/logout. If your API session is inactive for 35 minutes, the session expires and you are automatically signed out.
See API partner login for information about the additional permissions you get with the API partner login.