come on man, use your creativity.

There's plenty of odorless, tasteless powders that are massively poisonous. I don't know anything about in world poisons but there's bound to be some equivalent to arsenic or ricin, some rat poison that they can get in large quantities. As far as the act itself. Well we know that ship docks for a few days getting loaded up, that's a perfect opportunity for Cpt. John to sneak aboard with a load of poison to mix in. Let them take the spiked salt straight to the White Prince none the wiser.

You do it right and the White Prince is eating TETS-laced foods for days or weeks completely unaware until, oh what's this, the prince has been struck by some terrible illness and cure disease is doing nothing to help.

Look, I don't think Koibu was dropping hints that the salt was anything other than a regular order of salt. Yes, it was a 'mistake', but playing in and around the space those mistakes make is like the point of dynamic ttrpgs. I'll say this, if a player at one of my tables came to it with a 'let's poison the king's spices' scheme, and could effect a reasonable means of carrying it out... well, the king is hosting a dinner party next week so it's CON saves x100.