If you have an SSL certificate setup in WHMCS it is recommended to force all WHMCS pages to redirect from http to https. In this tutorial I'll show you just how easy it is to setup.
First you must have your SSL certificate installed, if WHMCS is installed in a sub-directory then install your SSL on domain.com and it'll cover any sub-directory. If WHMCS is installed on a sub-domain you'll need to install it for that particular sub-domain.
Next you'll need to set your WHMCS system URL to https:// in WHMCS (setup > general settings):