Android - 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.
There is the activity
Google Search > com.google.android.voicesearch.fragments.reminders.EditReminderActivity
which does exactly what you want.
Don't know if you found an answer or not but here is what I did. It requires Desktop VisualizeR.
Open Chrome on your mobile device and search for "Google Now Add Reminder". This will initiate a reminder card through through Chrome. In Chrome Menu select "Share..." Then select "DVR save to clipboard". Then create the shortcut on your home screen using the Desktop Visualizer app.
Found an easier way...lol...Bookmark it and add a shortcut to your home screen.