Subdomain redirects to internal page and stays visible in address bar

Hello,

in subdomain multisite installation, there is a need to create unusual redirect. I need to configure:
– subdomain xxx.siteaddress.com, which redirects to siteaddress.com/contentsite (this is usual Page in wordpress), AND
– xxx.siteaddress.com needs to stay visible in address bar.

This is internal configuration (subdomain to page in wordpress).

Could you please give a hint where this configuration could be done, without harming generic multisite configuration? (htaccess?)

Best regards,
wash