To add annotations to a score tap the icon half way down the right edge of the music on the Play screen .  The icon opens out to show the annotation tools.

Annotation Open   Open or close the annotation toolbar
Annotation Lines   Line colors and thicknesses
Annotation Erase  Erase
Annotation Undo   Undo/Redo annotation

Removing the Annotation icon

The Annotation icon only shows on unlocked documents.  I you’d rather not see the icon tap the padlock upper right.