The surefire way is to remove the starter from the
engine and slide the pinion gear to the "engaged" position so that what you're calling the "crossbar" moves towards the solenoid as far as it can. Then you can engage the solenoid's hook (possibly sliding the hook as far as possible out of the solenoid) and slide everything together.
There are various possible shortcuts to eliminate the need to unbolt the starter from the engine, like holding the "crossbar" in the engaged position with a screwdriver or a piece of wire or string. They don't always
work.