custom field related to another

here we go again.

i created a custom field under contact (Customer ID). There we fill in our internal customer number. I want to appear this field with the customer number also when i write an invoice etc. Menas: I create an invoice and the customer number is prefilled from contacts. How do i connect the field under contact to a custom field also called Customer ID in Invoice for instance ? Like the system prefill all other datas company name, street etc.


searched the whole forum. found some examples but it dont work or im simply to dump. that it is so difficult to copy one field to another module i didnt expect. Im tired. Ill give upt and look for a easier system