[Cross-posted in Technical Support]
It seems that the spell checker assumes it knows all the correction possibilities for what I have mistyped. However, not only does it not likely,* were it to allow me to input an unlisted correction as my current software does (and put the substitution in the Substitutions list), it could expand for me the shorthand I import from less-friendly typing/input media and devices.
It would be use-full had the spell-check dialogue a 'Replace with' input box for the sake of the unlisted corrections (and my expansions).
*Being effectively about one-month new to Scrivener and still in transition mode, I cannot be certain of the full accuracy of this statement: I'm somewhat going by past experience with the robust checkers of three other text/document apps.
I know that I could 'Guess' (per 15.2.3 in the manual, but maybe not yet in Windows

And thinking of the spellcheck dialogue, it would be nice were I able to to use keystrokes to get around in it and to 'Alt-key' select buttons and suggestions. (Keystroke navigation would also make getting into 'Guess' more fluid in the '*' instance.)