Here's some sample code...

code:
Break On CLS

$Form = CreateObject("Kixtart.Form")
$Form.Size = 220,100

$Combo1 = $Form.ComboBox(,20,30,200)
$Combo1.Top = 5
$Combo1.Left = 5
$Combo1.Additem (@WKSTA)
$Combo1.OnClick = "fnChangeBox()"

$Combo2 = $Form.ComboBox(,20,30,200)
$Combo2.Top = $Combo1.Bottom
$Combo2.Left = 5
$Combo2.AddItem (UserID1)
$Combo2.AddItem (UserID2)

$Form.Center
$Form.Show

While $Form.Visible
$nul=Execute($Form.DoEvents)
Loop

Function fnChangeBox()
$Combo2.Clear
$Combo2.AddItem (@USERID)
EndFunction