Update Add-MailboxPermission.md

Added changes made to mailusers

Author: ChandraKavya

exchange/exchange-ps/exchange/mailboxes/Add-MailboxPermission.md

@@ -15,7 +15,9 @@ monikerRange: "exchserver-ps-2010 || exchserver-ps-2013 || exchserver-ps-2016 ||
 ## SYNOPSIS
 This cmdlet is available in on-premises Exchange and in the cloud-based service. Some parameters and settings may be exclusive to one environment or the other.
 
-Use the Add-MailboxPermission cmdlet to add permissions to a mailbox.
+Use the Add-MailboxPermission cmdlet to add permissions to a mailbox. 
+
+Use the Add-MailboxPermission cmldet to add permissions to a mailuser in Exchange Server 2016, Exchange Server 2019 and Exchaneg Online.
 
 For information about the parameter sets in the Syntax section below, see [Exchange cmdlet syntax](https://docs.microsoft.com/powershell/exchange/exchange-server/exchange-cmdlet-syntax).