Office Plugin - Unable to display custom alert message for OnMessageSend

Question

Hello,

I'm currently developing an Office plugin for Outlook using the OnMessageSend and SmartAlerts events that will perform a few simple checks before the email is being sent.

I'm able to display custom alerts on the taskpane but i would love to be able to display them on the send button.

Something like this:

Any idea how to show this custom alert when i press the send button?

thanks