Usage patterns
〈人〉の許しを得るto obtain someone's permission
I gave the presentation after getting the teacher's permission.
許しを与えるto grant forgiveness or permission
He forgave his son for his mistake.
〈人〉に許しを求めるto ask someone for forgiveness or permission
She asked her parents for their forgiveness.
Expressions
許しを請うbeg forgiveness; ask for pardon; ask for permission