How can I update a custom field based on the Reporter field of an issue?

[trthomas]

In Assets I have a Department object that lists all of our organization's departments.  It is tied to a People object list that uses Department as one of its attributes.  

The Reporter field in the issue uses the People object list to show who opened the issue.  

I am trying to find the department of the Reporter (People object list) and then update a multi-select custom field on the issue with just that one department.

Could you please help with the JSON code that I will need to:

Find People.Department and update field Department (Multi Select) that is on the issue. 

Thanks