Make a customer field available as list in Case Form

I want to create a list of customer post codes that are contained in the customer form - address sub tab - post code field. I want this list to be available in the case form so after I select a customer, I can then select a postcode (from the drop down list) that I want my case to be associated with.

I have tried creating a custom field but cannot source the customer address postcode field - any ideas how I can do this - I thought this would be pretty simple to do,

Many Thanks

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