This feature synchronizes existing WHMCS clients to Mautic and adds them to previously mapped product and domain segments. If you have not yet mapped the products to your Mautic segments, do so before you synchronize existing customers.
If Mautic already have the client e-mail they will be merged with the exsting history of Mautic Contact.
Click on the button Synchronize existing clients to initiate the sync.
Before sync make sure that will have mapped the desire product to one of yours Mautic Segments.
Do not leave this page until all clients are finish to sync