in this link there is my code, how I did it by C#: community.dynamics.com/.../224137.aspx
using JS:
you can follow this example but it is not mine and I don't know if this works
bingsoft.wordpress.com/.../create-email-activity-from-template-using-javascript
BUT I wold take a different path...
you can retrieve the template with a simple soap call then you copy the
template.body field to the email.description field and it's done!
if i was not clear enough or if you need more informations feel free to ask.