Definitely! You can decide within Fastmail’s settings[0] which domain you want to use for masked emails. It can be fastmail.com, one of their fun domains like afcrichmond.uk, or one of your own. I've even seen some 1Password coworkers buy a brand new domain purely for their masked emails, so you can generate a “[email protected]” while still using “[email protected]” for your actual personal email.
[0]: https://www.fastmail.com/settings/domains