Unable to log in using GoogleApps ID

Unable to log in using GoogleApps ID

Under the application settings I have selected 'Sign in using GoogleApps', and I've entered my domain name. When I try to open Zoho Creator Helpdesk from a test account, it still prompts for login credentials. 

Ideally we do not want our users to have to enter any additional information just to access the ticketing system.

How can I fix this?

Thank you