Populate Manager DN

I have the following Situation:

I have users, that have an Extension Attribute called "ExtensionAttribute1" where on the user  the Manager employeID is saved (come from HR).

Now i want to search the appropiate Manager where "ExtensionAttribute1" = employeeID of the Manager.

Then write the found Managers DN in the users Manager field.

Someone has a clue how to do?

appreciate your help