Instrument change - staff display controll

In my orchestral score I have players, who have more than one instrument. How can I have controll on which staff Dorico displays in the score, when the bar is empty?
See the example and look at piano part:

I would like to have standard 5-line staffs on first bar.
Or – other option – I would like to have this 3-line staff also in third and fourth bar.
How can I achieve it?

Could you enter a hard rest in measure 1 of the 5-line piano part–or hidden text?
(Just guessing here.)

I tryed with shift-x text, playing techniques and dynamics, but it’s not working, even if it’s not hidden. What is hard rest and how can I input it?

I think you will find that you can’t influence the position at which the instrument change occurs: adding empty text items or explicit rests won’t make any difference, as it’s notes that determine where an instrument change occurs. You could try adding a stemless note and making it transparent.