You need to pass in the server auth code, redirectURI, and the player's display name to GameSparks. The server auth code and display name are unique to the player logging in, so the Google Online Subsystem will provide these. The redirectURI is in the game play services credential you set up on the google play console. You should receive a pop up asking you to sign in with google play - once you have logged in you can send the GooglePlayConnectRequest.
NOTE: Please ensure you are using https, not http, for any addresses. Otherwise, this would violate the underlying security policies on the endpoints in question, returning an error.