Show the Internal ID and External ID of the record in a custom field

1. Navigate to Setup > Customization > Entity Fields > New, enter the Label.

- Entity Fields for entity records such as Customers, Vendors, Contacts, Partners, etc.
- Transaction body fields for Transactions such as Sales Orders, Purchase Orders, Work Orders, etc
- CRM fields for Tasks, Events, Cases, etc

2.  Set Type as Free-Form Text and Store Value as False
3. On Applies To tab select Customer and under Display > Subtab select Main
4. Under Validation and Defaulting check Formula checkbox
5. On the Default Value field enter "{id}" which is the tag for Internal ID
6. Save
 
Note: For External ID > create another custom entity field > then on the Default Value field and set the default value as {externalid}