What you're wanting is the conditional drop down selection list. You already have your two parent tables set up: Customer List and Customer Contacts.
The relationship would go like this:
Customer List >> has many >> Customer Contacts.
Customer List >> has many >> Proposals.
Customer Contacts >> has many >> Proposals.
In Proposals you will have Related Customer List, and Related Customer Contacts.
In the Proposals table, go to the properties for Related Customer Contacts and establish Related Customer List as the parent to be selected first.
Be sure that when you are creating the Customer List records, that you select the appropriate Customer Contact lookup that is its parent.