I manually populate that field via a script I run against my HR report monthly. The field for "managed by" requires input (whether it be via powershell, or other) to be in a specific format. I believe that the CN or username must be used. It is an odd field that does not populate with static data. For example, you will get script errors if you try to add a name to that field and the account your are trying to insert does not exist in AD. The "managed by" field must contain users who exist if you want to script.