Generating a unique number for every submission using java or html

I have a signup form that I created using the plugin wpforms in wordpress. It was created for an event where people will physically come to the location, so instead of verifying them by names, I want everyone to have a unique id that can use to enter the event.

The question is: wpforms accepts html fields so.. How can I create a unique generated number, that will change every time someone refresh or open the page, so when someone enters his details, and submit it, his submission and data will be associated with that unique number(id) and sent to us.