Appointments Plus link to subdomain

In my theme, appointments plus puts into the main menu navigation at the top a link to its self created page that I called Make an Appointment, however, with the way my site is built, there is no acceptable font color/size scheme that works properly with the layout. So I want to have the navigation link point to a subdomain that has a different theme that is compatible with the layout and function of the appointments plus plugin.

My question is this: How to I get the Make an Appointment link in the navigation to point to my subdomain address, rather than a wordpress page in my main site? I have tried adding PHP code to this page to redirect to the subdomain, but it fails and is just displayed as text on the main site page.