How do I add an event on block touched?

You use the .Touched event. This code kills the player when they step on the part.

Part.Touched:Connect(function(Hit)
    if Hit.Parent:FindFirstChild("Humanoid") then 
        Hit.Parent:BreakJoints()    
    end
end)

Hit is the object that hit the part, we can use an if statement to check to see if the object's parent has a humanoid and if it does then it might be a player, to check if it is a player we can do

Part.Touched:Connect(function(Hit)
    if Hit.Parent:FindFirstChild("Humanoid") and game:GetService("Players")[Hit.Parent.Name] then 
        -- code
    end
end)

https://www.robloxdev.com/api-reference/event/BasePart/Touched