onchange Property
Returns or sets a Variant that represents the value of the onchange attribute that is the script to run when the onchange event fires.
expression.onchange
expression Required. An expression that returns one of the objects in the Applies To list.
Example
The following example sets the onchange attribute for the first TEXTAREA element in the active document.
Dim objTextbox As FPHTMLTextAreaElement
Set objTextbox = ActiveDocument.all.tags("textarea").Item(0)
objTextbox.onchange = "script()"