Auto Fill Columns when adding using New from Dropdown

I have 2 Tables - Orders and Products .

In Orders, I have a column for Product which is referenced to Product Table. So When adding , I use the dropdown to add the Product if not listed , user can use the New from the dropdown . All work ok but I have a column called First Order in the product table which is Referenced to Orders Table, So when a user creates a new order using the Product dropdown when creating an order, It should autofill the current orders ID in the First Order Column of the Products Table. How is it possible ?

Also , I dont see the New button in any dropdowns when editing exisiting records. What do I do to see the new button link when editing the record too just like when creating a new record.

@Steve - Can you help me with this or if you want I can try explaining more if its not clear ?

What is the purpose of the First Order column of the Product table? The first order for that product ever? Of the day? For the current user?

The first order of that product forever.