How to add Checkbox Fields to User Accounts?
You can add checkbox list from Account Settings

Go to  Configuration >> People >> Account settings - http://example.com/admin/config/people/accounts/fields

Add Field >> Add a new field of List(text)



Add options and select options as unlimited (for multiple select)



Save and go to Manage Form display (/admin/config/people/accounts/form-display) change the checkbox widget to checkboxes/radio buttons.



That's it you can see the check boxes in user account form like this