Javascript will not be fired when drop down is changed

I try to complete a foreign webpage with VB.NET

<select name="standardCode" class="form-control full-width" id="standardCode">
<option value="">Please select RSS</option>
<option value="510">Part1</option>
<option value="583">Part2</option>
</select>


<script>
        $("#saveAndContinueButton").on("click", function(){
            saveNames();
        });
     
        $("#standardCode").on("change", function (event){
            let standardCode = this.value;
            cleanSelect($("#version"));
        });
</script>

The “click” event can be fired by simple “ .getElementByID("saveAndContinueButton").click()”.

But the Drop Down Menu doesn’t work.

My code is able to select the item at the drop down, but the java will not be fired.

Sub Drop_Down()
        Dim doc2 As IHTMLDocument2 = IEApp.Document
        Dim dropdown As IHTMLElement = doc2.all.item("standardCode")
        Dim dropdownItems As IHTMLElementCollection = dropdown.children
        For Each element As IHTMLElement In dropdownItems
            If element.innerText = "Part1" Then
                element.setAttribute("selected", "selected")
                Dim doc3 = DirectCast(IEApp.Document, IHTMLDocument3)
                Dim doc3el = DirectCast(doc3.getElementById("standardCode"), IHTMLElement3)
                doc3el.FireEvent("onchange", Nothing)
            End If
    Next

I have no more ideas to solve.