How do I assign a keyboard shortcut to an AppleScript I wrote?
It is also possible without paid 3rd party apps!
1. Open Automator.
2. Make a new Quick Action.
3. Make sure it receives 'no input' at all programs.
4. Select Run Apple Script and type in your code.
5. Save!

Now go to System Preferences > Keyboard > Shortcuts. Select Services from the sidebar and find your service. Add a shortcut by double clicking (none).

Finally go to System Preferences > Security > Privacy > Accessibility and add Automator and the preferred app to run the shortcut.