Source Customer Internal ID on Contact Record.

To source Customer's Internal ID into the Contact Record please follow the steps below.

1. Navigate to Customization > Lists, Records, & Fields > Entity Fields > New.
2. Enter a Label.
3. Set Type to Free-Form text.
4. Umark the Store Value checkbox.
5. Mark Show in List.
6. Click Applies To tab > mark Contact.
7. Click Display tab > in the Subtab menu select Main.
8. Click Validation & Defaulting tab > Set Default Value to {customer.internalid}, mark the Formula checkbox.
10. Click Save.

View a Contact record with associated Customer record and users will see on the Main field the Internal ID of that Customer record.