You can add calendar reminders to any existing contacts, as well as edit them. 

  1. Once you are logged into your app, go to 'All Contacts' and click on the contact for which you want to add calendar reminder: 
  2. As long as you agreed for the app to use your native calendar feature, the events will be displayed on your default calendar: 

More information on how to add events, click on the link: How to create events using the calendar function