Make one netlify site a subdomain of another netlify site with a custom domain

I’ve found some other posts related to this but it doesn’t seem like they address what I’m trying to accomplish, or at least not in a way that I’m understanding.

I have one site, ethdefalliance.netlify.app with the custom domain https://ethereumdefensealliance.com/ using netlify nameservers. I then have another site govwatch.netlify.app which I want https://govwatch.ethereumdefensealliance.com/ to present.

I tried adding an alias for the govwatch subdomain to point to govwatch.netlify.app but that doesn’t seem to be working (see images below).

What am I doing wrong? How do I go about accomplishing this?

ethdefalliance.netlify.app details:


govwatch.netlify.app details:

You can add the subdomain govwatch.ethereumdefensealliance.com directly to the govwatch.netlify.app site and a record will automatically be created.