Overview#Login_hint is an OPTIONAL parameter in the Authentication Request as a "Hint" to the Authorization Server about the login identifier the End-User might use to log in (if necessary).
Login_hint can be used by a Relying Party (OAuth Client) if it first asks the End-User for their e-mail address (or other identifier) and then wants to pass that value as a hint to the discovered authentication server.
It is RECOMMENDED that the hint value match the value used for discovery. This value MAY also be a phone number in the format specified for the phone_number Claim.user it is trying to authenticate, it can provide the Login_hint parameter as a hint to the authentication server. Passing the Login_hint suppresses the Account Chooser and either pre-fill the email box on the sign-in form, or select the proper session (if the user is using multiple sign-in), which can help you avoid problems that occur if your app logs in the wrong user account.
The value can be either an email address or the sub string, which is equivalent to the user's Google ID.
The use of this parameter is left to the Identity Provider (IDP)'s discretion.