multi-domain - how do I set up root sites for other domains

I have set up Multi-domains and it is working.
I can set up new blogs and choose which domain they will show up on, and that all works.

But the root of the other domains all resolve to the root of the main domain.

In other words
WP is installed on
domain.com
a blog here works: antotherdomain.com/blog2/

how do I set blog to work at anotherdomain.com/