I have asked Spark if they could provide the same kind of URL on Mac as they do for instance if you use the built in service share. Spark also provide to create a link, but that is a Weblink, meaning anyone with that link will be able to see the email, which is deffintly not OK in many cases.
You can use the naitive Mail.app and drag-and-drop into Agenda, which works perfect, and support all platforms.
I would also prefer if spak and Agenda could find a solution for the Spark service menu.