Cannot add subdomain as alias for existing website?

Other Information

So i have a account ( Website for ronb.xyz ). When I try to add my subdomain, its directory is set to “routiney.prax.to/htdocs”. This means complete new site. I wanted it to serve as alias for ronb.xyz. but there seems to be now way to do it.

But when i am adding a custom domain, “prax.to”, i can choose the directory. here, i can set it as alias for “ronb.xyz”. but i cannot do this when adding subdomain?

Is there any way i can use my subdomain as alias to a website?