Custom application fields allow your organization to track responses to questions that matter to your organization across jobs.
After you have created a custom application field, the question will appear as part of any pre-existing candidate application.
Since the custom application field was created after the candidate submitted to the job, you will have to manually enter their response into application field.
To manually record/responses to custom application fields for existing candidates, open the Application section of the right panel on their profile, and scroll down to Application custom fields.
Click the Edit icon next to a custom field.
Enter the new value and click Save.