Window Set Focus

On click of a button i open a window. I am trying to set focus to the
window if that window is already opened. I am stuck as to what i am
missing.
The name of the window is "ShipRec" so i would assume it would be
ShipRec.focus() ; but i get an error.

Im missing something. Can anyone help ?

Thanks,

Miro

Example

function OpenScreenWindow(cScreenToShow)
{
if( cScreenToShow == "PMXParts" )
{
window.open('PMX/Parts.htm','ShipRec','height=625px,width=920px,too lbar=no,directories=no,status=no,menubar=no,scroll bars=no,resizable=no,modal=yes');//Code Required Here }}