You can not setup your hosting server with FORWARD URL.
When you forward or redirect an URL this mean that when the request arrive to JMS, the original URL is lost.
When a request arrive to JMS and that is not defined in its list of slave site domain, it displays the master website.
You have to modify the hosting server to define a sub-domain without redirection or forwarding but with an assign of its directory.
See the FAQ
www.jms2win.com/faq that contain a section that will help you configure your hosting server depending on its type.
As you says CPanel, probably you should have a look on the FAQ
/faq#sec-58 and
www.jms2win.com/media/joomla_multisites/config_cpanel.pdf
See also the video present in tutorial step 0
www.jms2win.com/tutorial that explain the concept of JMS and that highligh the point concerning the hosting server confgiuration.