Disable User Registration on Open Source install

Hi guys.
I am new to the topic of LoRaWAN.
I have the TTN v3 Open Source edition installed on an Ubuntu server.
Everything works without problems and the first applications are running.
I just do not know now how to enable the option that strangers can not register with my instance?
There is an option “is.user-registration.enabled” on the Identity Server.
But how can I set this option to “false”?
I would be very grateful for any help here.
I didn’t find a suitable topic in the search unfortunately.