Notification to supplier when contract is shared

Summary:

No notifications are sent to the supplier when a contract is shared. Is there a way to set a notification on that event?

Content (required):

A notification should be sent to the supplier when a user checks "Share externally" option in the overview tab. Otherwise how else would a supplier be notified that the client has shared the contract.

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):