Default value in a multiselect inherited from a field in another module

Hi all

I think I´ve seen an answer to this but can´t find it now - maybe I just dreamt it 

I have a multiselect field on the Contacts record (field B).

The same dropdown list is in a field on the Accounts record (field A).

When creating a new Contact, I want to give field B a default value that is the same as in the field A (on it´s Accounts related record). Basically field B = field A at that point.

How can I achieve taht?

Thanks,

KGM