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