The user name doesn't have much to do with your domain name. It's just an FTP login. I've had to resort to variations on a name to find one not taken. Like if you have smith.com, and you try to create a user 'smith.' Probably taken. Try bobbyjoesmith. Then you can proceed to create an email address of email@example.com, which is a completely different entity from your FTP user.
If you want to create a donations.example.com subdomain that's "owned" by bobbyjoesmith, that works, too. Again, end users will never see the "bobbyjoesmith" username.