How do I create a REST API key?

To create REST API keys, click on the menu item API under Settings.
The creation is done via the green button at the bottom right, Create REST API key.

Assign a name, e.g., the name of the application that uses the key.

You can allow all IP addresses or only enable certain IP addresses, which you can enter here.
To enable a range of IP addresses, use * as a wildcard (XX.XXX.XXX.*).