rather than having that configuration be done through glitch, an alternate approach is to make a new glitch app for what you’d use a subdomain for. For example, api.myproject.glitch.me becomes myproject-api.glitch.me.
Additionally, I’m not 100% sure on this, but with url mapping you may be able to simulate subdomains based on seperate projects.
so if you map
myproject.glitch.me to www.myproject.com
you may be able to also a separate project to an arbitrary subdomain