In QMC I've added a custom property and set the resource type to "Extensions" and also added a value to it.
Then in my extension I selected the value for the custom property.
Now to the problem: I haven't found any way to actually access that custom propery from the extension and read the value.
The custom property is a URL to a server. The idea is to setup up different URLs for each customer installation of Qlik Sense. Then the extension can use the URL to send data to the correct server.