@robert-regnier Unfortunately, it looks FusionAuth does not support this. I did find an
open issue that you may want to upvote if it is in fact the same issue you are experiencing.
You may also want to check out this blog. It talks about a custom domain using one of FusionAuth's hosting packages, but think it should work in your case as well.
Does anyone else have a similar setup? How did you implement it?