Instead of using DialogAPI onSend event, there is the simple way to achieve exactly what you need and display the prompt which is called "Smart Alerts". There is a difference between Smart Alerts and the on-send feature. There are a lot of resources on "Smart Alert" feature of Outlook add-on. The following article is the great way to start: Handle OnMessageSend and OnAppointmentSend events in your Outlook add-in with Smart Alerts. The example of the implementation, similar functionality you are asking about, but for attachment check can be found Automatically check for an attachment before a message is sent.