This is a menu item class setting:
Configurator | Sales tab | Menu Item Classes button | Options tab. Reference Required is the option you're looking for.
Unless this is a specialty item there are probably more than one item linked to that class. You're better off copying the class, selecting that option and then linking the item to the new class.
Quick tip - most forms in the configurator have keyboard shortcuts. To copy a menu item class just select it, press the F5 key and then F6. It will insert a copy of your class as the first available number.