Credential ID field and External Credentials checkbox is not shown on the Credential form after enabling the External Credentials Store plugin (com.snc.discovery.external_credentials)
Out of box, the External credential store checkbox is only configured to show on the External Storage form view. Similarly, the Credential ID field will appear once the External credential store checkbox is checked.
If you switch your form view from default to External Storage view, you will notice that the External Storage Credentials checkbox becomes visible.
You can do this by:
1. Navigating to the credential record
2. Right clicking on the context menu
3. Choosing View > External Storage