You could install your own webmail software in a subdirectory of your domain if you wish, and then “disable” the default “DH provided” webmail from within the control panel.
You can set-up your DH provided webmail to use some subdomain other than “webmail.yourdomain.com” from within the web panel, but I see no way to use the panel change it to use a subdomain.
Finally, you could setup a redirect so that a user typing in www.yourdomain.com/mail, or even just www.yourdomain.com/m, would be "automagically forwarded " to the webmail at webmail.yourdomain.com or some other subdmain you set up in the control panel. While I don’t really see the point in doing that, it would make “it available on another URL”.
Why do you want it at that other URL? Would a redirect work for you?
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