Voting button automation

I have created a MailItem using vba with a Calendar Item attachment and added a voting button with .VotingOptions = "Add to Calendar". I want the mail recipient to be able to press the "Add to Calendar" button and have the Calendar Item added to their personal calendar. When I did a similar routine from an Outlook form, I used the Function Item_CustomAction to detect when the voting button had been pressed and set the Item_CustomAction = False so that a new email was not created. How to I detect the action in vba and NOT create a new item?