In order to set up required guest details for online check-in go to Property Settings / Online Guest Account.
Below the buttons you will find the list with the guest details for booking owner and you can decide which of them are required, active (optional) or which should be inactive.
See: What is the difference between a “Booking Owner” and a “Guest” once they are checked in?
If a booking has more than one guests they also will be able to enter their details.
You can decide if the required fields for the other guests should be the same as in case of booking owner:
Or different than the ones required for the booking owner:
Don't forget to save the changes!