If 'Open a list of URLs' is selected as the startup action, this allows you to specify the list of URLs that are opened. If left not set no URL will be opened on start up.
This policy only works if the 'RestoreOnStartup' policy is set to 'RestoreOnStartupIsURLs'.
This policy is available only on Windows instances that are joined to a Microsoft® Active Directory® domain. or Windows 10 Pro or Enterprise instances that enrolled for device management.