Hi and welcome to the forum! You can only use free subdomains on free hosting, exceptionally on Premium Hosting if you migrate your website to them, then nowhere else. You can consider getting your own domain if you want to point it anywhere else; here are some places where you can get one for free:
The 403 Forbidden error being returned by the control panel is quite crude, but the first screenshot shows a very clear issue:
You are trying to setup a CNAME record that points to a URL. That doesn’t work. CNAME records need to point to a hostname, not a URL. That means there should not be any http:// prefix, or any / at all for that matter.
If you would rather redirect your subdomain to a particular URL, please use the Redirects tool.
What exactly are you trying to do? If you can tell us more about what you’re trying to do, maybe we can help you figure out the best way to do that.