I have asked Spark developers to add a service that would enable a “save to clipboard” option for the backlink to a message, or another method of generating backlinks and working with them directly. (To be clear, this is different than creating a weblink to view a message on the web – this is the kind of link that would directly open the Spark app on your device to the email in question.)
Currently, they have integration with services such as Things, OmniFocus, and some others, which create, e.g., a task with the backlink recorded in the note field. You can copy the resulting link and paste it into Agenda, but obviously this is clumsy. If you agree with me that having a more direct access to backlinks would be useful, please email Spark support using the “Send Feedback” menu to add your vote to the idea – hopefully, it will motivate them to work on this in the near future.