// Add a checkBox item to a form and require exactly two selections.
var checkBoxItem = form.addCheckboxItem();
checkBoxItem.setTitle('What two condiments would you like on your hot dog?');
checkBoxItem.setChoices([
checkBoxItem.createChoice('Ketchup'),
checkBoxItem.createChoice('Mustard'),
checkBoxItem.createChoice('Relish')
]);
var checkBoxValidation = FormApp.createCheckboxValidation()
.setHelpText(“Select two condiments.”)
.requireSelectExactly(2)
.build();
checkBoxItem.setValidation(checkBoxValidation);