Hello all,
I am trying to integrate an application using the external ID in Zendesk. When I login through zendesk, I can see the external ID on the tickets I am working on:

Yet when I access through the API the external ID is set as null:

I am using the tickets API: /api/v2/tickets/ticket_id
Any help on this?
Are you referring to the external ID that is present in the user profile card to the right of your screenshot? If so, that is the user's external ID - not the ticket's external ID. You can side-load related users when getting a ticket via API to return the requester's user info (including their external ID). Ex.
/api/v2/tickets/{id}.json?include=users