This blog has How to generate an email instance inside the new 1402 Activity BO – Workaround as a prerequisite.
You are able to send the email. Now you want to add this email to the related items of an activity from wich is generated.
Just follow the highlighted code!
The code is inside a custom BO action called by a webservice. The emailSubject webservice parameter contains the raising activity UUID, and is used to retrieve the raising activity instance.