Token Endpoint for OAuth2

Hello, I am working with @Alberto Fuentes https://openexchange.intersystems.com/package/workshop-iris-oauth2.

I wonder how do I get a token to send to the resserver?

I tried calling "token_endpoint" : "https://webserver/authserver/oauth2/token"

The response was "error":"invalid_request","error_description":"Unknown grant_type"

I wonder what needs to be in the request body? Is it a GET or POST?