To find the ID of an email, go to the edition of that email. In the URL of the browser, you will see a 6-digit number, which represents the idMessage.
https://app.dialoginsight.com/gw/contact/template-visual-editor/editor/message/732439?idSample=9#dndedit
The idMessage in this example is 123456.
The idMessage is required in web services calls that trigger email campaigns.