Add as alias or url

https://support.google.com/a/answer/7502379?hl=en

https://support.google.com/a/answer/175747?hl=en

Add aliases to your user if url

Add aliases to your gmail via settings

BEWARE! You will loose access to calendars created with a google account linked to that domain before you thought about the aliases. You will not be able to use your aliases as the creator of the calendar events.