Replace Firebase Project Name with Custom Domain in Email Templates with Action URL

It is possible change the Firebase action URL in email templates to one’s custom domain without implementing email action handler code but relying on the already existing firebase functionality.

In the Firebase Console:

1. Go to Dynamic Links

2. Add URL Prefix by entering your custom domain YOURDOMAIN.com. No need to actually add a Dynamic Link, this entry alone is sufficient.

3. Go to Authentication > Template > Edit (Pencil) > Customize Action URL

4. Enter your custom domain according to this pattern (note the TWO underscores):

https://YOURDOMAIN.com/__/auth/action

5. Send out an email by registering to your App and click the link in the email. The link will bring you to the default Firebase page handling the action.