MiaB requires its own domain and its own server.
If you have a domain with WordPress and want it to also be MiaB, you can have MiaB use that domain and then use the DNS server in MiaB to configure an external DNS A/AAAA records for wherever WordPress is located.
Or you can use a second domain for MiaB and then configure the domain you have on WordPress to use MiaB for mail on that domain’s DNS server.