The value in the cell has to be Text (not a real number, not a formula).
So preformat the cell as text and type 23 into that cell
(or add a leading apostrophe '23)
select the 3 in the formulabar
format|cells|check Superscript.
This won't make the value 8--it'll be just text that looks pretty.
This technique won't work with real numbers. If you really wanted it to
evaluate to 8, you could use:
=2^3