Ah, I see what you're asking. There is no place on the sheet to specifically keep track of maximum hit points. If you take a look at
Elfog Oakshoulder's sheet and scroll down you'll see that for my characters I keep track of things like that in the "Notes" section.
The "Temp HP" field is used for spells/conditions that actually give you extra HP for a certain amount of time and may come into play at some point in the future of our game.