Lyx Shortcut Math Function Sign (e.g. accepts item in A returns item in B)

To obtain an equivalent of ${}^AB$ in LyX, you can

1. start an empty math mode box,
2. type ^ to enter superscript and enter the A,
3. press space to exit the superscript,
4. enter the B immediately after the superscript.

Checking View > Source Pane, the above produces $^{A}B$, which compiles as ${}^AB$.

If it is something you need to insert many times, then create a keyboard shortcut: Go to Tools > Preferences > Editing > Shortcuts > New and as function use math-insert ^{A}B.