At “User Data Deletion”, choose the “Data Deletion Instructions URL” option, and enter the URL of your page* with the instructions on how users can delete their accounts on your site.
To comply with GDPR, you should already offer the possibility to delete accounts on your site, either by the user or by the admin:
If each user has an option to delete the account: the URL should point to a guide showing the way users can delete their accounts.
If the accounts are deleted by an admin: then you should have a section – usually in the Privacy Policy – with the contact details, where users can send their account erasure requests. In this case the URL should point to this section of the document.