You can add Co-Applicants, Co-Signers, or Occupants after submitting — but only while your application is still showing as Submitted, before the property manager has started the review.
Before review starts
On the "Your Application Is Submitted" confirmation page, tap Go Back & Make Changes
Go to the Cosigners or Occupants section
Add the new person
Re-submit your application
Going back will pause the property manager's review until you re-submit, so do this promptly.
Once review has started
If your application has moved to Under Review, you can no longer add someone yourself. Contact the property manager — they can add the person directly from their dashboard.