Display the Internal ID of the Customer in Case Record

Below are the steps on how to create a custom field that will pull up the customer Internal ID in case record:
 

1.  Navigate to Customization > Lists, Records, & Fields > CRM Fields > Click New.
2. Set Internal ID on Label field.

3. Set Type to Free-Form text
4. Uncheck Store Value.
5. Mark Show in List.
6. On Applies To tab > Mark Case.
7. On Display tab > Subtab field choose Main.
8. On Validation & Defaulting tab > Set Default Value to {companyid}

9. Mark Formula checkbox.
10.  Click Save.

Note: The value will pull up upon saving the record.