One can pull the template ID used in the 'Is Sent Sales Email' -> Template ID constraint from the URL when you have the MSI email selected in Design Studio.
For example, if your URL is https://app-sj##.marketo.com/#EM45322B2, your Template ID is 45322.
This method has been used for sales reps to automatically kick off a follow-up drip campaign after they send the first email.