You can do this a couple of ways, by checking the property Occupant of the seat itself, check the SeatPart of the players Character, or use @OIogist way.
Seat Script Example:
local Seat = script.Parent -- Path to seat
Seat:GetPropertyChangedSignal("Occupant"):Connect(function()
if Seat.Occupant ~= nil then
local Char = Seat.Occupant.Parent
Char["HumanoidRootPart"].Anchored = true
-- Can also set their jump power if wanting, your preference
end
end)