tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page. No catch. No spam. No wares. It's truly free and always will be.
Copy files modules folder to your whmcs installation folder. After that you will have to activate it by going to Setup -> Addon modules (admin/configaddonmods.php) and finding and activating Tawk.to widget module. Then make sure that you configure access to this module so that you have access to it.
If you don't have tawk.to account, you can always create one for free