How can we fetch candidate id when an application is submitted or when application status is modified?

How can we fetch candidate id when an application is submitted or when application status is modified?
Currently we only get applicant id with the webhooks data. - https://developers.pinpointhq.com/reference/get-application

I added a custom field to the application, is there any way I can automatically fill this custom_field with the candidate id to the fetch the candidates. Also, I feel candidates are nothing but job seekers. Please support on these 3 queries.