I don't believe its a form for a record on a table per se, but I do believe it references "email_client.do" as the form it frames the window with, when using that feature. I'm not sure how you access it to get the code or copy/customize/insert&stay/etc...I'll update the thread here if I figure that out.
Note that it's called the "Email Client" (Email client (servicenow.com)), so may be difficult when searching for answers -- as "Email Client" isn't a very unique string to look for, when referring to something specific.
Having said that, you can enable this exact functionality for any table in the platform by adding the email_client dictionary attribute on the specific table's collection record...so if you are just wanting to USE this on a table which isn't Incident, you don't need to re-create it. Here are some instructions for that: https://docs.servicenow.com/bundle/sandiego-servicenow-platform/page/administer/notification/task/en...
