Link subdomain to a folder of root IP

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How you do it will depend on what you want to happen. You could simply set the desired folder as the document root for the name-based virtual host for that subdomain. This will serve the content in that folder when visiting the subdomain in a browser.

You could configure a redirect rule for that hostname and when people visit that subdomain they will be redirected to the specific folder at the other site. This method will show the the address of the main domain once the redirect completes.