2021bryant
You won’t find any train saying “Leuk” on the exterior displays nor departure displays mentioning “Leuk”, as you’ll need to additionally change trains up to 3 times in either Zurich HB (City), Berne or Visp.
Simply look up the available connections using the online timetable on the SBB website, or install and use the SBB Mobile app. Enter Zurich Flughafen as “From”, Leukerbad (Busterminal) as “To” and check out the available connections. If drilling into the connection details (i.e. the first leg of the journey), the departure and arrival platform numbers will always be indicated.
At the ZRH Airport station located underground next to Check-in 3 area, your trains would all be on platforms 3 or 4 anyway (same escalators down), since all eastbound trains will arrive on those two. It is not more difficult to find than any departure gate, just follow the signs with a railway symbol.