First you create your button and your subform,In the subform, in the
object->subform tab, in the presence option, select hidden.Select your
button, and in the script editor, choose formcalc as the language. Then,
you select the event Click. In the editor, you click as if you want to
write in it, then you hold CTRL and click on your subform. This will
write the path to this subform in the editor. Then after the path you
write :path.presence = 'visible' (don't write majuscule on visible or it
won'...