If you want to use a workshop collection on your server (either your own collection or a third party one) you will need to create a new command line to launch the server.
For that go on your gaming panel then click on the command line changer icon.
Now click on new to create a new command line, you will come to this page:
You will need to give a name (top) without spaces or special characters then activate the boxes and fill at least the fields presented in the screenshot by of course setting the ID of your collection and the corresponding authentication key.
Save the new command line and return to the command line changer homepage and select it.