To allow all participants enter to the promotion, the clue is always to share the main link, the URL which is compatible with mobile devices.
This link is unique for every promotion and it is shown on the promotion management page, outlinded in a green box.
Note: Facebook doesn't show in mobile devices the app tabs where the promotions are located. For this reason, it is important not to use the link you find on your browser address toolbar, because is the promotion tab address. If you spread this link by mistake, mobile users won't be able to enter or participate at the promotion.