The Proof of Identity section is found on the Applicants Page, at the bottom of an Applicant's details:

Click on the three dots on the right of the Drivers Licence field:

And a pop-up will appear to allow you to add the detail of the Drivers Licence:

You will note a tooltip on the page that gives you some further detail about acceptable Driver's Licence numbers:

Once you have entered the detail, click save and you will note the Proof of Identity section is 'complete':

In Supporting Documents, you will see a checklist item for a copy of the Drivers Licence:

Which you are able to upload and drag across to satisfy the condition: