Well, I'd assume you make it a toggle that activates if you hit.
Right but what if you want to save your smite for when you Crit, not just hit? Now Technically, Smite isn't specifically part of the reaction system since it doesn't expend your reaction, but its considered part of it anyway since - you know - its a decision that occurs within an existing action.
Monks have a similar thing, and there is also the matter of Counterspell...
Imo, not being able to save Smite for crits is fine. In Solasta, it's mainly the psuedo-reactions (smite, metamagic) asked after every attack that can get at all annoying, so I'll take the ease-of-use in a toggle Smite in exchange for the loss of being able to crit-fish smites with a pop-up.
Plus, you can rest whenever in BG3 (and the game encourages you to rest after ~anything significant to get cutscenes), which restores all your slots. So there's not a
need to save slots up for crits - just smite on every attack against an enemy with more than ~10 hp and then long rest after the fight.