Once you've completed the Rent-To-Own program for your Circle2 plugin, you can find your serial key here: https://splice.com/plugins/your-plugins. Your Circle2 plugin will continue to run with the Splice app open.
If you want to use your plugins without the Splice app, you can register your serial number by:
Launch the Circle2 plugin and click on the FAW logo at the top left:
You will be able to enter your lifetime serial number and use Circle2 offline without Splice.