Add a mandatory field to be completed when creating an SO - to capture the lead source

It would be very beneficial to create a mandatory field with a drop down menu for the staff to select from when creating a Sale Order?

We are wanting to specifically capture if the order was received via our Adwords Marketing Campaign or via our Website online form.

We may want to have up to 5 options in the drop down menu to include Account Customer, Walkin, Google.