To connect RosterBuster to your iCloud you will need to generate an app-specific password and use that instead of your regular iCloud user details. You will need to enable multi-factor authentication for your Apple account if you haven't done so already.
Apple's help article on how to do that can be found here:
Apple required that all applications use app-specific passwords when connecting to iCloud services. Therefore generating an app-specific password became mandatory.
When you have generated the app specific password for iCloud you can use this to link your RosterBuster account.
1. Open our website www.rosterbuster.com
2. Go to menu Settings / Calendar
3. Tap enable iCloud integration
4. Provide your Apple ID and the generated app specific password