Call a Public Sub when a Flag is clicked on in the Message Preview pane

Outlook version
Outlook 2016 32 bit
Email Account
Exchange Server 2013
Hi All,

I am trying to Call a Public Sub saved under a module in my Outlook Project 1, via a Sub in my ThisOutlookSession. But have had no success.

What I need is to trigger the macro in a module with name - Public Sub SetCustomFlag(), when I click on the Flag icon in the message priview pane

The macro Public Sub SetCustomFlag(), works fine by itself in a module, and when I call the macro externally. And now I am trying to automate the process, everytime I click the flag icon on the message preview.

Thank you for your valuable suggestions.
Outlook version
Outlook 2016 32 bit
Email Account
Office 365 Exchange
This macro at Create a Task when a Message is Flagged shows how to do something when a message is flagged - i do everything in the macro in thisoutlooksession - but could split it into two - the macro would call the second macro like this:
SetCustomFlag item

then name the next macro
Public Sub SetCustomFlag(item as mailitem),