Piano Forum

Good question!  Never expected to see "ad libitum" in a Mozart sonata.
I haven't studied this with a Mozart scholar, and there isn't much on the internet, so I asked an AI chat. It said:

"In musical notation, "ad libitum" means "at one's pleasure" or "at liberty." In the context of measure 198 in the third movement of Mozart's Sonata K.333, "ad libitum" suggests that the performer has some freedom to interpret or modify the music according to their personal taste.
This could involve variations in tempo, dynamics, or even embellishments, allowing for a more personal expression of the music. In classical music, this kind of instruction gives the performer room to add their own interpretive flair while maintaining the overall structure of the piece."

Embellishments mean trills or turns, etc.  It's a few measures long so I would guess Mozart intended it to be played as written - in terms of notes.  I would be surprised if there are any recordings out there with additional notes - perhaps someone knows of one.  But I think you should feel free explore adding to it - improvisation is an important skill for a pianist.  I'm sure Mozart would have added notes in a performance, if he felt inclined.