Verify licensed users/clients using email address?

[inspiro]

How do I verify clients using their email address the way it's done on this forum? My clients details are stored in WHMCS. This is the API: http://wiki.whmcs.com/API:Get_Clients_Details

Thank you so much for your help.

[Xencored]

AdminCP >> Opitons >> User Registration Options

Verify Email address in Registration = Yes