Put one on top of the other. Make the id field based on a value list of Customer ID & Customer Name and enterable in browse mode. Make the Customer name field only enterable in find mode.
There's a check box to select in Manage | Value list that hides the ID's from the first field when the value list deploys.
If you select the option to use this value list as a pop up menu, radio buttons or check boxes, you will not see the ID number even when you are not selecting a value from this value list.
For a drop down list, you will see the ID number once you exit the field. You can, however, hide this drop down list formatted field behind a name field that has Browse mode access blocked via a behavior setting in the Inspector.
And other options are possible as demonstrated in "Adventures in FileMaking #2 - Enhanced Value Selection", where you'll find a more advanced way to hide the ID field as well as a way to use a "name only" value list to link records by ID - with full support for adding new records for values not in the list and handling cases where there are identical names.