Motion practice comes later.
All this "Einleitung" business in German falls away, just extra. We do sometimes say "institute" or "commence" proceedings, but in a different context, for instance, "We reserve our right to institute proceedings before the ___ Court, etc."
I would also question whether this "tribual" is not simply an arbitral tribunal. In that case, it would simply be Request [or Demand] for Arbitration.