+screen = Builder.load_string('''
+BoxLayout:
+ orientation: 'horizontal'
+ FormCanvas:
+ id: form_canvas
+ BoxLayout:
+ size_hint: 0.2, 1
+ orientation: 'vertical'
+ Button:
+ text: 'Add button'
+ on_press: app.add_button()
+ Button:
+ text: 'Add label'
+ on_press: app.add_label()
+''')
+