Here's the screen-by-screen flow of the licensing process.

It starts with the above window and directs users to the website to pick and purchase a license code, which is given on spot and also sent in the receipt email.
Then copy-paste the code, click on Verify, get this -

If this works (the server is accessible, the license code is valid, etc), then this is it. Thank you very much, click Ok, you are done -
Otherwise, it explains the failure and offers either to retry the activation or to use an alternative -
There are two alternatives, both are simple -
For offline boxes and locked-down setups there's the Copy To Clipboard option. Take the URL to a networked machine, open it in a browser, get a licensing file back and then drop it into the app's config folder.

The URL is exactly the same as the one used by the app's own licensing code and it comprises of the license code and installation ID and the fingerprint.

Prepare the email option creates a new email with default email client and puts the Installation ID, Fingerprint and License Code in the message along with some pleasantries. The license file will be attached to the reply and it will simply need to be dropped into %LocalAppData%\Bvckup2\engine.