There are, essentially, two ways to license MC:
1. From within the application, you can restore the software by entering your license key in directly into the application (either via the Trial splash screen or under the Help > Install License menu option). This method "uses up" one of your "restores" and requires an active Internet connection on the machine.
2. When you do Option #1 above, the system will email you a Temporary Installation Key (which is a MJR file attached to the email). This file can be used to license ANY copies of MC you want, and they don't need an Internet connection (and it does not use up a "restore"). You simply need to copy this file over to the PC somehow and double click it (MC19 must be installed and working beforehand). This will open and register the license in MC19. These MJR files "last" for 14 days from when they were generated (after which they expire and you must generate a new one).
If you don't have ANY PCs where MC is installed and you also have an Internet connection, you can generate one of these MJR files manually on any Internet connected computer by visiting this page:
https://rover.jriver.com/cgi-bin/restore.cgiThat will generate the MJR file, which you can download right through the browser, and it will also email the file to you at the email address you used when you purchased MC originally.
So, generate one of these MJR files, if you don't already have one recently generated sitting in your email inbox, and then sneakernet it over to the offline PC (USB stick or whatever), and you'll be good to go.