capture/read the Table id from the header on the window screen

OK... first things first... go to https://www.w3schools.com/js/default.asp and take the basic tutorials on writing JavaScript code... this request this time is probably day one high-school programming course.  If you're going to be doing automated test development, I would STRONGLY recommend taking online tutorials or classes to get familiar with basic programming techniques.

That said... here's how I would write it

var captionString = OrderEntry2.WndCaption; //This returns the value of the property
var hyphenPosition = aqString.Find(captionString, "-"); //This finds the position of the hyphen
var tableIDLength = hyphenPosition - 7; //We know the starting position is 7 so simply subtract that from the hyphen start
var tableID = aqString.Trim(aqString.SubString(captionString, 7, tableIDLength)) //Use Trim and Substring to get the actual TableID
Log.Message(tableID)

 Code is tested and functional based upon a WndCaption in the format of Order #123456 - IPO