Create a field for Line Manager in a new request

Create a field for Line Manager in a new request

Hi - apologies if this has been asked, but I can't find the answer!! I am trying to create a field for Line Manager for a new joiner type of service request. To ensure it is correct, I was hoping that I could make the field query the list of requestors within ServiceDesk plus, which sync's from our AD, but can't seem to figure out how to do it. Any ideas?  Thanks!