Linking to WHMCS
From WHMCS Documentation
It is very common for users of WHMCS to want to link to WHMCS from their website. This can be in the form of an order link, or with a form that either takes the user straight to the domain checking process to check the availability of the domain they want or to login.
With WHMCS, this is made easy as all the code you need is provided to you customised and ready to insert into your website by simply going to Utilities > Integration Code.