Help with mastodon please?

Been trying to link mastodon and write.as for a month now. I really battle.

On write.as my handle is @blog@mydomainname.com
I can change the @blog part, but I can not change the @mydomainname.com part
I clicked the links to create a mastodon account.
the handle is @mynickname@writing.exchange
I can’t seem to get @blog@mydomainname.com whichever way I try.
How do I make my write.as talk to mastodon?
Thank you!