You can add to backlog with the personalised email too

Chad Dyer if you use drag and drop to add the task or if you click on a calendar date under the "add to today" button in the preview you should be able to assign the task to any day you wish.

As far as adding it to the backlog there is an indirect workaround for this where you initially assign the task to some day and then move it from the day to your backlog. Not super elegant but it works.