You should set a variable (like PlusWnd) which gets the return value of MsgPlus.CreateWnd, like:
code:
var PlusWnd = MsgPlus.CreateWnd( 'windows_new.xml', 'Polite', 0 );
Then, you get the line from the file:
code:
var filePath = MsgPlus.ScriptFilesPath + "\\myfile.txt";
var sLine = new ActiveXObject('Scripting.FileSystemObject').GetFile(filePath).OpenAsTextStream(1, -1).ReadLine();
Then, you set the value using SetControlText:
code:
PlusWnd.SetControlText("EdtWelcomeMsg", sLine);