1. Navigate to Setup > Customization > Entity Fields/Transaction Body fields > New, and 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 asFree-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 the 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}“