Answer
Bound actions don't let you pass an entity-type parameter explicitly in the workflow designer, which is why the Call activity doesn't prompt for one. Instead, add an API Read activity that reads the exact record you want to act on first (including any VirtualSet key it needs), then add a Call activity for the bound action with no parameters — the workflow implicitly uses the record from the preceding Read as the action's target. This also applies when the bound action is exposed on a VirtualSet-based entity rather than a normal one.