If-need-be is automatically enabled for all group polls and cannot be turned off. This gives you and your participants the highest level of flexibility as it allows your participants to select an option and indicate that this options works for them if necessary, but it's not their first choice. This will help the organizer decide which option works best for all the participants.
When the participants vote, all they have to do is click an extra time on the selected option for it to be marked as "if-need-be" (yellow). To remove it, they can click on it again which will deselect the option (light blue), and then click once more to select it as "yes" (dark blue).
This is how a if-need-be selection would look in the group poll