Hi by using the developer mode  how can i create a related field from another custom field and here is my eg :

- i create a new custom field named ( object ) on sale.order, so  after creating an invoice i should find the same value for the ( object ).

i already made an a related field from another existing field ( name ) by adding partner_id.name on related field.

But for a custom field i found some difficult on what i should put in the case of Related field, so i hope  that the community help me to find the correct way thanks.