I thought he wanted to change the link for the Support Center so that when he clicked Support Center it would go to his WHMCS page.
Jeff what you described works when you click "Create a Ticket" it will take him to his WHMCS page.
So I guess I misunderstood the question. All he said was "how can i change this link" so I thought he meant the Support Center link not the Create a Ticket link.
EDIT: To explain a little more using Jeff's suggestion you end up clicking Support Center, then Create a Ticket, and then you are presented with your custom link to WHMCS.
I was thinking 2 levels up.