You've mixed up the terms a bit...
First of all, the part after the http:// is called the hostname. A Hostname can be a domain or a subdomain, it is not called a subdirectory.
Second, "folders" and "directories" are the same thing but directory is the more accurate term since a directory is simply a list and "folder" is just a metaphor for GUI-ness.
Also, the part of the URL that comes after the hostname is called the URL-path.
If you want the URL to stay the same (which is what you mean if you say "remap") then you need to set the document root of the subdomain to the filesystem path of the subdirectory.
- Get the document root path of domainname.com from the Web Panel - it is called the "Web Directory"
- Append the url-path to this path
Example: '/home/user/domain/' becomes '/home/user/domain/subdir/'
- Set the document root of the subdomain to this path
If you want the URL to change, then edit the subdomain to be "Redirected" instead of "Fully hosted" and specify the new URL instead of a filesystem path.
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