you will find in the Tool bar Tab (Category) <Work>. The button (Icon) you will find in the Tool bar Tab (Category) <Work>.
Mark the area which you want to fill with rests. Then click on the corresponding button:
The parts of the voices (empty bars) will be filled out with the rests.
It can go over several bars, staves and voices!
Alternatively, you can find this function in menu "Tools">Edit Notes/Selection>Fill with the rests