CmdID / PowerShell / Cmdlets / Send
Send-RDUserMessage
Sends a system message to a specified user session. Windows Server PowerShell reference
From the Windows Server PowerShell reference 4 parameters Its page in the reference
Parameters
-HostServerSpecifies the name of the server that hosts the session.-MessageBodySpecifies the text for the message body.-MessageTitleSpecifies the text for the message title.-UnifiedSessionIDSpecifies a unique session ID on the host.