At Sendios, we add the user ID to the link as a parameter u: https://n.site.com/link/?p=16986&u=1080010303 . If you follow our lead, you will find the user in your database.
4. Ask the user to display the user’s email in the app settings and take a screenshot.
Your developers receive a private relay email when the user logs in to your service. As a result, they can reveal it in the app settings. So, the user will be able to copy it and send a screenshot to you.