(in a normal .rc file)
SysTrayOnAdd !Systray_Add
SysTrayOnDel !SysTray_Del

(in a script file)
trayXpos $ResolutionX/2$

*script bang !Systray_Add
*script exec !varAdd trayXpos -9
*script exec !SystrayMove %{trayXpos} -30
*script ~bang

*script bang !Systray_Del
*script exec !varAdd trayXpos 9
*script exec !SystrayMove %{trayXpos} -30
*script ~bang