To enable passkey upgrades in Google Password Manager, place a JSON file on your server at /.well-known/passkey-endpoints. This is called a "passkey endpoints well-known URL" and is an open protocol for aligned parties to formally advertise their support for passkeys and provide direct links for passkey enrollment and management. You can expect a similar effect on other platforms when they support passkey endpoints well-known URLs.

For more information check out: Promote passkey upgrades in Google Password Manager