I m a Reseller web host.I want to redirect my clients’ TTs. How can I redirect my clients’ TTs to another help desk?

You need to customize the templates (menu.xml file in the common/ files)
You will have to create 2 custom pages: one for TT creation, located at
common/tt/new_tt_form.html
the other one for viewing TTs, located at
common/tt/user_view_tts.html
In the HTML body of your custom pages, redirect the client to your support system by linking to the required URL.