Thx Richard H, Jooel, Shawn and Les 
It works splendidly now, here's the final code:
(keep in mind, the dialog is very plain and simple)
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, $iControlMessage
$sMessengerTitle="Messenger-service"
$iControlMessage=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)
; Timer to catch messages 
$Timer = $System.Timer
$Timer.Interval = 1000 ;1000ms = 1s
$Timer.Tick = "CatchMSG()" ;the event that fires every second
$Timer.Start
; 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())
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
  run "net send $rcpt $msg"
  $TextBox1.Text = ""
  :end
EndFunction
Function CatchMSG()
Dim $sMessageText, $sFrom, $sDate, $sTime
  If $oAutoIT.WinExists($sMessengerTitle)
    $sMessageText=$oAutoIT.ControlGetText("last","",$iControlMessage)
    $=$oAutoIt.WinClose("last")
    $sMessageText=split($sMessageText,@CRLF)
    $sFrom=Split($sMessageText[0]+"      ")[2]
    $sTo=Split($sMessageText[0]+"      ")[4]
    $sDate=Split($sMessageText[0]+"      ")[6]
    $sTime=Split($sMessageText[0]+"      ")[7]+" "+Split($sMessageText[0]+"      ")[8]
    $sMessageText[0]=""
    $sMessageText=SubStr(Join($sMessageText,@CRLF),5)
    $log = $log + @CRLF + $sFrom + " zegt tegen jou:" + @CRLF + $sMessageText + @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