it's possible now with the nightly version (might need negative X on some machines)

  local wezterm = require("wezterm")
  wezterm.on('gui-startup', function(cmd) -- set startup Window position
    local tab, pane, window = wezterm.mux.spawn_window(cmd or
      {position={x=0,y=0},width=100,height=20}
    )
  end)