Learn how to enable the extension ID field.

1. Navigate to Setup.
2. Under the desired object, navigate to Fields.
3. In Salesforce Classic, for standard objects click Customize > Object name > Fields. For custom objects, click Create > Objects > Object Name, and then scroll to Custom Fields and Relationships.

   In Lightening experience, click the Object Manager tab and select Object Name > Fields & Relationships.

4. Select New.
5. Select Text and click Next.
   
6. In the Field Information section, in the Field Label, enter ext_id.
7. In the Field Name field, enter ext_id.
8. In the Length field, enter 255.
9. In the General Options section, select the External ID - set this field as the unique record identifier from an external system checkbox.
10. In the Text options section, in the Length field, enter 255.
11. Select Next.
12. From the list displayed, select the profiles that should be able to view this field.
13. Select Next.
14. Choose the page layout where it can be viewed.
15. Select Save.