How to add FiveM Server license key authentication key.
First visit https://keymaster.fivem.net and generate your server key.
Select VPS or Dedicated Server
At the bottom of your configuration file add this lines and edit enteryourkeyhere to match your key:
#license key for server (https://keymaster.fivem.net) select VPS or Dedicated Server
Click on SAVE Button
Click on Updates and install latest update then Restart your Server
NOTE: You must use one key for one server !!!
If you get following error open Support Ticket
GlobalError: This server does not have a license key specified. Please set the sv_licenseKey console variable to a key from https://keymaster.fivem.net/. (for example, `set sv_licenseKey "key"` in the config)