Messagebox not displaying to prompt user to close application

I am writing a PS worklet to prompt the user to close Outlook to allow the install of the Mimecast for Outlook plugin. I would like to not go down the road of creating a scheduled task to perform this action. 

I have tried a couple different options:

Send-ActiveUserMessage -Title 'Close Outlook' -Message 'Outlook will close automatically in 1 minute for the Mimecast plugin Install. Please wait 5 mins before opening Outlook again. Thanks, IT.' DialogType 'OK' -Timeout 60

&

# Add required assembly for MessageBox
Add-Type -AssemblyName "System.Windows.Forms"

# Show MessageBox that Outlook is closing
[System.Windows.Forms.MessageBox]::Show('Outlook will close automatically in 1 minute for the Mimecast plugin Install. Please wait 5 mins before opening Outlook again. Thanks, IT.','Close Outlook','OK','Warning')

Neither seem to display an interactive message for the user but work in ISE. I read in a previous post here that this was due to the worklet being run as system user and that makes sense, but an AutoMox employee mention the Send-ActiveUserMessage as an option.

Has anyone had success here and can share what they used?