...Never tried this, but I would imagine you could either try to detect when the player pushes the Shift-Key while riding an entity; Or, (and I think this would probably work better), make a procedure that executes every player tick, and checks if the player is riding an entity. (You should be able to find the 'Is riding entity' block in the entity data tab.)
...Never tried this, but I would imagine you could either try to detect when the player pushes the Shift-Key while riding an entity; Or, (and I think this would probably work better), make a procedure that executes every player tick, and checks if the player is riding an entity. (You should be able to find the 'Is riding entity' block in the entity data tab.)