In 2018 it was enough to show the pass onboard (no mandatory reservation)

I don’t understand if you booked reservation or not. If yes, you don’t have to connect the reservation with your pass.

Seat reservation on Inlandsbanan is optional, but if you don't have a reservation you can only travel if there are free seats. As Inlandsbanan often is fully booked, in practice, this means that you need a seat reservation to travel with Inlandsbanan. 

In addition to the seat reservations you also need to show you the ticket (QR code) from your Global pass. You need to add the journey with the spelling of the stations used by Interrail. You can also look for the stations on the map in the app. 

I can't find the Inlandsbanan train in the planner either so I guess that you will need to add the journey manually.