One possible way of doing this:
Enter Opt. as staff-attached text followed by a space. After the space, right-click and select Insert Music text… . In the dialog which opens, enter ottavaAlta in the Search field and click OK. If you need a line such as the one which normally shows the extent of the 8va, you will need to choose the appropriate line from the line palette in the right-hand zone, and then align it manually in Engrave mode.
