Assignments - Closing Comment - Can I make this required?

Hi,

I have been tasked with making the closing comment of an assignment required. I am not sure how to do this. Does anyone have any ideas?

Another thing I have been tasked with is modifying the permissions of the deadline property to only be editable by the creator of the assignment, the assignees manager or their one over manager. I also don't know how to set permissions to just one property rather than the whole object.