If I understand your post correctly, you want a value list of addresses, but only those for the current customer that are also identified as "Main"?
Or do you just want to select the customer and see their "main" address appear automatically?
I want to select the customer to see their main address automatically. A customer has many different types of addresses but I only want the type that is labelled as main to appear.
So if you have a relationship:
Customer::__pkCustomerID = Addresses::_fkCustomerID
then you can put a portal to addresses on a layout based on Customer and use a portal filter to limit the addresses records to only those identified as "main". If there is only one such address possible, this can be a one row portal. Please not that you don't have to use a "table like" single row of fields in this single portal row, you can resize the portal row and position fields within the row like it's a small form. And you can make the borders of the portal invisible so that the user doesn't see anything but the address fields.
Your portal filter expression might look similar to this:
Addresses::Type = "Main"