Sorry for all the comments you're getting. You can absolutely do what you're trying to do.
The technical answer is you first have to setup a subdomain. Then, using webfinger, you can point that subdomain back to your primary domain. This will make your Mastodon server appear as your primary domain even though your website / email is already parked there.
It's explained in this blog post on Masto.host. I had someone else set this up as I'm not technical, but it absolutely works.
Sorry for all the comments you're getting. You can absolutely do what you're trying to do.
The technical answer is you first have to setup a subdomain. Then, using webfinger, you can point that subdomain back to your primary domain. This will make your Mastodon server appear as your primary domain even though your website / email is already parked there.
It's explained in this blog post on Masto.host. I had someone else set this up as I'm not technical, but it absolutely works.
https://masto.host/mastodon-usernames-different-from-the-domain-used-for-installation/