command /test [<text>]:
trigger:
open chest with 1 row named "&7Testowe gui" to player
wait 3 tick
set player's current inventory's slot 0 to diamond named "&b&lDiament"
on inventory click:
if inventory name of current inventory of player is "&7Testowe gui":
cancel event
No chyba, że tak bardzo chcesz tego format slota...
command /test [<text>]:
trigger:
open chest with 1 row named "&7Testowe gui" to player
wait 3 tick
format slot 0 of player with 1 of diamond named "&b&lDiament" to be unstealable