To handle licensing is not a easy task.
1. I would build a database hosted on my server with a table that holds your license and a start och end date (could also be timestamp).
2. From your solution I would setup a Insert from URL step that asks your license database on your FMS if user that access your solution have a valid license at start up of there solution. To make this work in the long run, you would have to set a Idle time on your FMS so that all users that are inactive for example 4 hours automatically are logged out and need to log in again to run your start script where you check whether your license is valid or not.
Apologies, I missed a vital piece of info. The license is for a standalone RunTime solution (RTS). The user requests a license by completing name fields etc, exports an FM file, emailed to me, I paste in the encrypted info, and email them a license which they paste into a field. For the RTS to activate, that 'license field' must match the hidden data within the RTS.
It's not worth the expense of hosting yet.
Runtime will disappear from the FileMaker family in later versions of FM. Maybe you should look into other ways of letting your users use your solution already know and handle the license in the way I wrote about earlier.