The more common way of handling this situation is to avoid it. Your customers should have a unique, FM-generated auto-enter serial. Then you would create a calculation ( result is text) in your customers table which concatenates name and phone (or street address) such as:
LastName & ", " & FirstName & " " & Phone
Then set up the value list to insert the unique ID into the foreign key CustomerID field but display name also of the calculation so User can select the correct Customer. Once the CustomerID is in the table, the name of the customer can be displayed directly from the Customer table - no need to automatically 'fill in' the first name. By using primary keys in this way, it eliminates redundant information, i.e. the customer name should only ever appear in one place ... the Customers table.
If you instead insert the customer name again in this other table and the customer name is incorrect (or the customer changes their name), you would have to change the spelling through all your records. Each bit of data should only ever be entered once in a database structure. :^)
But then I have to enter a customer number in order to retrieve the name, right?
Thanks everyone. I figured it out.