Hi, @lifin. I’m going to send you a private message (PM) as well with a few additional details.
However, I can explain 100% of this solution publicly. When you are not using Netlify DNS (and this domain does not use it), there is a manual step our support team must do. We have a support guide about this here:
Aso, there is one other detail. The branch subdomain will be under the “primary domain” for the site.
So, if the branch is staging and the primary domain is example.com, then the brand subdomain will be staging.example.com.
However, I believe this set has the www subdomain set as primary. Using the example.com example above, now the primary domain is www.example.com. This makes the branch subdomain a subdomain of that domain so it would be staging.www.example.com not staging.example.com.
Again, I’m going to PM some additional details but please feel free to reply here.