Shift+lclick on a potion in your inventory - it goes automatically in the belt if there is a potion of the same type in some column and free space above it. If it is a mana/healing/rejuv potion, it will go to free column if there is no space left in other columns with the potions of the same type.
(It's basically the same as if you pick up the potion)
When shopping, shift + lclick a potion - it buys all potion of that type and fills your belt according to the above rule. (With complete empty belt, you can buy 16 mana potions in 1 click).