xdotool

See ArchWiki's Openbox page for details.

Notes

1. Personally I like to employ this pattern in tint2 using the executor- for one this allows you to spawn a different menu when the icon is right clicked, and two it allows some text to accompany the icon (which doesn't seem possible with the tint2 launcher).
2. If you opt for, say, tint2 using the launcher method as originally described on ArchWiki, ensure you DO NOT enable startup notifications by default in your launcher. With tint2,

   $ cat ~/.config/tint2/tint2rg | grep startup
   

   should return

   startup_notifications = 0