When creating a slot, you need to specify which trucking company you are driving for. You can only choose companies that you are already connected to in the passify app.
If the company you need is not listed, add it first:
Go to Settings → Personal details→ Trucking company
Send a connection request to the company you are driving for
Once accepted, the company will appear as a choice during the pre-registration
You can be connected to multiple companies at the same time and simply select the correct one when booking each slot.