Currently, there is no way to make an add-on required. Therefore, anything required to book your space should be priced into your base rate and detailed in the Rules section of your listing.
Remember, guests expect access to the space and amenities highlighted in the content of your listings, including photographs. Add-ons are intended to allow you flexibility to sell premium amenity upgrades. They are not meant to be an itemized upcharging for basic amenities, like restrooms. Learn more about what you should and shouldn’t charge for here.