301 Redirecting domain back to subdomain.mainsite.com

I have a subdomain WP multisite network with domain mapping

one of the domains is expiring. I want to 301 redirect the domain back to the multisite subdomain and keep those pages active.

I don't really care about individual pages, just redirect the whole thing to one address

domain.com
subdomain.mainsite.com

I have the WP Redirection plugin installed but everything I've tried results in a loop so looking for an htaccess solution - but no clue about how to write that.

ideas? thanks