It is highly recommended to use an SSL on your domain in order to enable https:// (instead of insecure http://). With WHMCS it is easy to enable your SSL 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.
To enable your SSL you'll need to set your WHMCS system URL to https:// in WHMCS under Configuration (Spanner Icon in the top right) > System Settings > General Settings > WHMCS System URL: