Some script I found in the new roblox studio template

local Seat = script.Parent.Seat
local ProximityPrompt = script.Parent.ProximityPrompt

ProximityPrompt.Triggered:connect(function(plr) -- upon clicking
    if Seat.Occupant then return end -- make sure seat isn't already taken

    local char = plr.Character
    if not char then return end -- make sure the player has a character

    local humanoid = char:FindFirstChildOfClass("Humanoid")
    if not humanoid then return end -- make sure the char has a humanoid

    Seat:Sit(humanoid) -- forces the humanoid to sit on the seat
end)

Seat:GetPropertyChangedSignal("Occupant"):Connect(function()
    local humanoid = Seat.Occupant
    if humanoid then
        ProximityPrompt.Enabled = false
    else
        ProximityPrompt.Enabled = true
    end
end)