How to get "Person or Group" Field to auto populate Department Field for name selected

I have SharePoint 2013 list that has a Person or Group field called "Intake Form Contact".  I have a second field called "Associated Department". I have created a Nintex Form and will be building a workflow next.  When someone uses this form and selects name from Person or Group field, how can I get the manager of that selected person to auto populate? Wondering if this can be done within form or with a workflow. Thanks in advance!