Class CheckboxValidationBuilder

// 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);