Registering LanSend activates your license on this computer. You buy a key and enter it from the Help menu.

Open Help > Get license key to go to the order page. Pick the license that fits how you use LanSend:
You can also buy a LanSend license directly from the website.
Once you have a key, open Help > Enter license key to open the Register LanSend dialog. Type the name the license is issued to in Licensed to, paste the key into License key, and click OK. LanSend saves the license on this computer.
A LanSend license key begins with the prefix LK1-LNS-, so you can tell it apart from keys for other products.