local width, height = minetest.get_screen_size();

local player = minetest.get_local_player();

-- ^ this is client-side script
-- for server-side (single player), we might do minetest.get_sp_player()

local formspec = FormSpec()

formspec.width = width / 2
formspec.height = height / 2

local button = Button()

button.text = "OK"
button.width = 75
button.height = 25
button.x = (width - formspec.width) / 2
button.y = (height - formspec.height) / 2

button.onclick = function(target, params)

    -- target - ref to button which was clicked (because the same callback can be
                bound to multiple buttons)
    -- params - additional mouse/keyboard parameters

    target.parentForm:close()
end

formspec:addControl(button);

formspec:showTo(player); -- or maybe player:showForm(formspec)