Get user who checked a checkbox and add the user in Relate field(Users module)

Hi, I have 3 fields: Checkbox, Relate field & Date in my custom module. All users can access the module but only few users can edit or update the fields. Here, when a user logs in and checked the checkbox, I need the get the user and update him in the relate field. Also I need to get the date when the checkbox is checked and update it in the date field. I didn't find calculated value option for Relate fields. Can this be possible using Studio? or do I need to make any code customizations?