I found a way to toggle STAMINA mode with Tasker.
In Tasker, just create a new task of type "Send Intent". Then in "Action" field, enter :
Then exist Task.
Now, when you enter this Task with a profile, STAMINA mode is toggled on or off. You can check this by adding the STAMINA widget on your home, or by looking at the "+" sign near the battery indicator.
Hope this helps !