how to get the delegated person By who delegate on the calendar in the workflow?

My user on vacation would delegate other user on the calendar.

The delegated person didn't have the right to complete the task.

I want to write a method for getting the delegated person and dynamically increasing permissions.

But I have a problem to get the delegated person by the user on vacation.

I already get the user on vacation.

Checking network and community, but still have no found.