IF you want one and only one check box selected in the field. Use Set Field to set the value for that check box just as though this were a simple text field.
If you need to select more than one check box or you need to add that selection without losing the data already selected, you'll need to use a more sophisticated expression.
This script step selects "new Record" while not losing any other values already selected in the field:
Set Field [yourTable::yourcheckboxField ; List ( YourTable::YourCheckboxField ; "New Record" ) ]
If there's a chance that "new record" might already be selected, you can keep the data in this field a bit cleaner (you won't see any difference unless you inspect the field without any check box formatting):
Set Field [ YourTable::yourcheckboxField ; If ( IsEmpty ( FilterValues ( YourTable::YourCheckboxField ; "New Record" ) ) ;
List ( YourTable::YourCheckboxField ; "New Record" ) ;
This expression avoids appending a second instance "new value" if it is already selected.