In general there are two possible approaches.
You can place a waypoint on the motorway if you only want to include a short passage on the motorway. Right-click on the motorway and select “set as waypoint”. In the app you can use a long press. Note that most motorways have two opposite lanes. It is important to place the waypoint on the correct side, if the result looks weird, you can try to move the waypoint to the other lane.
If you want to enforce a whole route on the motorway, you can use the fastest route, this should already result in a high likelihood to end up on the motorway. If this is not enough for you, you can try advanced settings like
avoid main roads and
avoid narrow roads to further force Kurviger on the motorway. This should result in a mostly motorway route unless there is no reasonable motorway along the route. Make sure not to select
avoid toll roads if toll charges incur on the motorway in your country.