Javascript to populate customerid field

Hi there,

I'm trying to do something quite simple but not being great with Javascript, I need some help.

On the case form, I have 2 lookup fields fields: customerid and primarycontactid.

When a user fills the primarycontactid, I need to auto-populate the customerid field with the contact's parent account.

I tried using workflows, but customerid is system required and needs to be filled before saving :(.

Any help is appreciated.