Default assignment category based on department dff field

Summary:

Default assignment category based on department dff field

Content (please ensure you mask any confidential information):

I am trying to write an expression to mirror an autocomplete rule we have currently,

The current autocomplete rule defaults a value into the assignment category field(we have renamed business area) based on the value which is stored against a flexfield on the department which is called business area.

Has anyone any ideas how I can write the logic so it works? My attempts so far are not working

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25B

Code Snippet (add any code snippets that support your topic, if applicable):