There is more than one way to do it. Some easier, some better. In each case, you must enable "developer mode" first. To do that, go to Settings / Dashboard, and look for "enable developer mode" link somewhere on the right.

• The harder is to create a module with code from Giusy.

• Easier is to customize the translation, finding and editing the label via Settings/Translations/Translated terms.

• You can also customize the form view by inserting a help string. But there is a bunch of inherited view in product form and the right one is a bit hard to find. Click on the bug icon at the top right of the screen, then on Open View. In the search box, type "product". Then click on the "product.product.form" (I guess it is the one you want) then bug icon again and "Edit Form View".
  

Hi!!

You must enter:

_inherit = 'product.product'

default_code = fields.Char('Customer Artwork code/Internal reference', index=True)