On the App platform I have setup a web service (API), a database and a static site.

Access to the static site is through: mydomain.com
The web service has the route: mydomain.com/api
I have the DNS managed by DigitalOcean

And all works well.

I now want to ADD a new static site to my app platform that I can access on the subdomain: portal.mydomain.com
However I cannot find out how to do this. I checked the question “How to setup subdomain with DO Apps and Namecheap?” on this platform but that does not answer my question or I do not understand the answer.
I did manage to setup the subdomain portal.mydomain.com, but that links to the static site for mydomain.com

Is this possible and how do I do this?