The referral page can be found in the dashboard at 'Wifi marketing' > 'Welcome page'. If you then click on 'Login methods', the page shown in the image below will appear. At the bottom is a 'Redirect page' heading. The page (website) you enter here is the page that will be displayed as soon as the Wifi connection has been established.
For example, you can refer to the Facebook page here. This means that more people will come to this page; so more likes!