@pixelastic, at this time the only way to link a custom domains to a Netlify site is by the automatic creation in the form of “branch name” + “primary custom domain name”.
If the site’s primary custom domain is example.com and the branch name is “branch-1” then the automatic custom domain will be branch-1.example.com. This same limitation applies to sites using external DNS instead of Netlify DNS as well. The naming convention above must be followed.
If you want to have a custom domain link to a branch of a site, that branch must be the production branch for the site.
So, using the example above, you would make a new Netlify site linked to the same Git repo as “treasurehunt” but where the production branch for the site was the branch “alpha”. One this was done, then add the domain as the custom domain for this Netlify site.
Again, at this time, there is no way to add arbitrary custom domains to the branch subdomains of single Netlify site.