I've been trying to get your code to work all day Richard but can't. (edited, removed obsolete code)
(Please note the "Messenger-service / Messenger Service" difference)
Here's a simple dialog I tried it with
Code:
Break On
$System = CreateObject("Kixforms.System")
If Not $System
   $nul= MessageBox("KiXforms.Net Not Initiated. This Script Will Now Close.","Error",16)
   Quit()
EndIf
$nul = $System.Application.EnableVisualStyles
; GLOBAL SETUP - PLACE AT SCRIPT START----------------------
GLOBAL $oAutoIT, $sMessengerTitle, $iControlMessenger
$sMessengerTitle="Messenger-service"
$iControlMessenger=65535
 
$oAutoIT=CreateObject("AutoItX3.Control")
If @ERROR OR VarType($oAutoIT)<>9 "Cannot create AutoIT object - no registered perhaps?"+@CRLF Exit @ERROR EndIf
 
; Set some useful options.
$=$oAutoIT.Opt("MouseCoordMode",0)
$=$oAutoIT.Opt("WinTitleMatchMode",4)
 
; END OF GLOBAL SETUP---------------------------------------
$Form1 = $System.Form()
$Form1.Left = 368
$Form1.StartPosition = 0  ;FormStartPosition_Manual
$Form1.Size = $System.Size(240,358) ;(Width,Height)
$Form1.Text = "LAN Messenger"
$Form1.Top = 0
$Label1 = $System.Label()
$Label1.BorderStyle = 0  ;FixedSingle
$Label1.Left = 24
$Label1.Text = "Gebruiker"
$Label1.TextAlign = 16  ;MiddleLeft
$Label1.Top = 19
$Label1.Width = 55
$nul = $Form1.Controls.Add($Label1)
$ComboBox1 = $System.ComboBox()
$ComboBox1.Height = 21
$ComboBox1.ItemHeight = 13
$ComboBox1.Left = 89
$ComboBox1.Text = "Select..."
$ComboBox1.Top = 20
$nul = $Form1.Controls.Add($ComboBox1)
$RichTextBox1 = $System.RichTextBox()
$RichTextBox1.Height = 171
$RichTextBox1.Left = 13
$RichTextBox1.Text = ""
$RichTextBox1.Top = 68
$RichTextBox1.Width = 209
$nul = $Form1.Controls.Add($RichTextBox1)
$TextBox1 = $System.TextBox()
$TextBox1.Height = 61
$TextBox1.Left = 13
$TextBox1.Text = ""
$TextBox1.Top = 258
$TextBox1.Width = 125
$TextBox1.Multiline = -1
$nul = $Form1.Controls.Add($TextBox1)
$Button1 = $System.Button()
$Button1.Height = 61
$Button1.Left = 152
$Button1.Text = "Send!"
$Button1.Top = 258
$Button1.Width = 70
$Button1.Click = "SendMSG()"
$nul = $Form1.Controls.Add($Button1)
$Form1.Show  ;Displays the Form
Init()
While $Form1.Visible
   $Nul = Execute($Form1.DoEvents())
   $RichTextBox1.Text = $RichTextBox1.Text + @CRLF + CatchMSG()
Loop
Exit 0
Function SendMSG()
  Dim $msg, $rcpt, $x
  $msg = $TextBox1.Text
  $rcpt = $ComboBox1.Text
  If $rcpt = "Select..."
    $x = MessageBox("You have to select a user first!","Info",0)
    Goto "end"
  EndIf
  $log = $log + @CRLF + "Jij zegt tegen " + $rcpt + ":" + @CRLF + $msg + @CRLF
  $RichTextBox1.Text = $log
  $TextBox1.Text = ""
  :end
EndFunction
Function CatchMSG()
  If $oAutoIT.WinExists($sMessengerTitle)
    $CatchMSG=$oAutoIT.ControlGetText("last","",$iControlMessage)
    $=$oAutoIt.WinClose("last")
;    $log = $log + @CRLF + $oAutoIT.ControlGetText("last","",$iControlMessage) + @CRLF
;    $RichTextBox1.Text = $log
  EndIf
EndFunction
Function Init()
	;ComboBoxbox 1 Info
	$Users = GetObject("WinNT://@ldomain")
	$Users.filter = "User",""
	For Each $User In $Users
	  $x = $ComboBox1.Items.Add($User.Name)
	Next
EndFunction