A simple check in doc - with a button that documents when a user checked into the event

I often get questions that need the user to modify a specific row.
“I have a list of participants in a table. I wanted to add a button on the main section to “Check In” a participant when they arrive and add the date/time to the next column. I’ve created it and it adds the date/time, but it does it for all the participants in my list. So I was wondering if there was a way to set the button to ask which participant?”

Here is a little example doc that uses a Select List to let you chose a Participant and then only modifies the row for that particular user.