Trying to get a sitting animation seat for only one seat, can someone help?

check when a child is added that is a weld and is named "SeatWeld"

script.Parent.ChildAdded:Connect(function(c) -- when a child is added (assuming the parent is the seat)
    if c:IsA("Weld") and c.Name == "SeatWeld" then -- if the added child is a weld and is named "SeatWeld"
        if c.Part1.Parent:FindFirstChildOfClass("Humanoid") then -- if the script finds a humanoid
            local anim = Instance.new("Animation") -- makes the animation
            anim.AnimationId = "rbxassetid://AssetIdHere"  -- sets the id of it
            local track = c.Part1.Parent:FindFirstChildOfClass("Humanoid"):LoadAnimation(anim) -- loads the animation
            track:Play() -- plays the animation
        end
    end
end)