After you have purchased FabFilter Twin in the online FabFilter Shop, you will immediately receive an e-mail containing your personal license key. This license key will turn the evaluation version into a fully registered plug-in.

After you have entered your license information, you will be need to restart the plug-in host, so make sure you save your sounds if needed. From now on, you will be able to use FabFilter Twin for an unlimited period of time with full support via email.
We recommend you to join the FabFilter mailing list to keep up with the latest updates.
See Also
Purchasing FabFilter Twin
Support
License agreement