Shortcut/Widget to set a reminder in Google Now
I don't think it's possible to add a shortcut to the "add reminder" card at the moment. I've had a look through the Activities as well and none of them appear to do what you want.

There is a "Set reminder" option in the menu within Google Now which may help? Click the three dots at the bottom right (you might need to scroll down) and you should see the "Set reminder" option.