How do you integrate Wordpress and WHMCS when they are on the same server?

I'm trying to integrate WHMCS and Wordpress which are both located on the same server.

My Wordpress installation is in /var/www/html making the URL, https://web.redskyhosting.com.

My WHMCS installation is in /var/www/html/portal-whmcs making the URL, https://web.redskyhosting.com/portal-whmcs.

How do I set the parameters of the plugin such that this could work?