You can do either. You can install the trial version of LightLogger Keylogger first and buy a license key later, or you can buy a license key first and then install LightLogger and register it with your paid key.
For the limitations of the trial version, see Can I try LightLogger before I buy it?
For instructions on installing LightLogger, see How do I install LightLogger on my Computer?
For instructions on purchasing a license key and activating a paid version of LightLogger, see How do I purchase a paid license key and activate it?Note: Do not buy a LightLogger license for any Windows version other than Windows 10, 8, 7, Windows Vista, or Windows XP. You cannot use LightLogger on other systems (for example, Windows 2000), and your purchase price cannot be refunded.Keylogger Home