Populate Search Dialog with a value from a TAB item

I'm trying to populate a search dialog with a value from an item within a tab. The value for "SID" always comes back with the PARENT Form "Buy Orders"

My method that is launched from vendor property on event "OnSearchDialog"

// I need the id from the "Part Number" that is selected in "Buy Parts Tab"
//var sid = top.thisItem.getPropertyAttribute("source_id");
sid = "2AE98CF00ADA400387B282E1AE2B734A"
var retObj = new Object();
sid = "2AE98CF00ADA400387B282E1AE2B734A"
retObj["source_id"] = { filterValue: sid, isFilterFixed: true };
return retObj;