It is clear to me now that I have not been clear
Indeed I want to fill a form (Promo_Distribution_Form) partly with data from tables. For what I would like to know, I believe 1 table is sufficient as example:
There are Shops that carry the same Shop_name, but are located in different places (and of course have different Shop_ID)
In order to fill this form I need to add the correct customer to identify the shop. In the first listbox I select Shop_name, when opening the 2nd listbox, I would like to see only the locations where shops with this name can be found, for me to select.
Basically it is indeed a filter function I am looking for, but can not find the way to solve this in the form.
In examples where you have 2 tables connected with a 1-n relation I could follow as the key was used to identify. Here there is no key for this as it is 1 table.