Select Specific Account When Sending Email, Based on Current Folder

Outlook version
Outlook 2016 32 bit
Email Account
Hi all -
I want to automatically choose a specific e-mail account when opening a new (blank) e-mail message from a specific folder. I have tried everything, but I cannot get my code to "fire" each time a new e-mail message is opened. I understand how to find the current folder, and I understand how to set the account to use, I just don't understand how to fire my code every time a new message is opened. I know it involves Inspectors, but I do not understand the logic.
Any help appreciated!
WIN7, 64