Assigning ortho toggle to F6 key

The Keyboard tab of the Customize dialog allows you to assign meanings to function keys and some other keys and key combinations. Just take what's in the Title and Command fields of the F8 shortcut and put it in the same fields for the F6 shortcut instead.

If there's no shortcut for F6, you can create a new shortcut by right-clicking on any shortcut and selecting "Insert shortcut." Then select the radio button for "Create new tool." Then type or paste in the Title and Command fields. Then OK. Then put your cursor in the Key field and press F6.