Update 2019
Starting with Adaxes 2019.1 it is possible to manage calendar permissions in Adaxes Web interface and Administration console without using scripts. Also, it can be automated using the Modify Exchange properties action. For details, have a look at the following tutorial: https://www.adaxes.com/tutorials_AutomatingDailyTasks_AutomateExchangeMailboxConfiguration.htm#collapse_cal_perms.
Original
Hello,
When accessing Exchange functions from PowerShell scripts, you need to use PowerShell remoting and access your Exchange server withing a Remote PowerShell session as recommended by Microsoft.
Here's how you can execute your command in a script launched by Business Rules, Custom Commands or Scheduled Tasks:
$exchangeServer = "ExchangeServer.com" # TODO: Modify me
$session = new-pssession -connectionURI "http://$exchangeServer/powershell" -ConfigurationName Microsoft.Exchange
Import-PSSession -session $session
Add-MailboxFolderPermission -Identity $Mailbox":\Kalender" -User $Benutzer -AccessRights $AccessType
Remove-PSSession -Session $session
In the above code snippet, $exchangeServer specifies the DNS name of the computer where your Exchange Server is installed.