How do you use autolayout to position a button at the bottom of a scrollview?

I have a viewcontroller that contains a view which contains a scrollview which contains my content i.e

ViewController:
- View > ScrollView -> text feild

This is so when I tap in the text field the content moves up and the keyboard doesn't cover the text field.

Now I come to use autolayout I add constraints for the scrollview for 0,0,0,0 for leading, trailing (to container), top, bottom (to top bottom layout guide). It duly fills the screen on a 3.5" or 4" screen. So for so good.

The problem comes when add a constraint for the text field to 5px for "bottom space to container." It does nothing. I would be grateful for any help to lay out objects in the scrollview.