This is my add form:
<AddForm>

<ScriptBlock BlockType="HeadScript" RegisterOnce="True" ScriptId="XMODPro_Admin">

<link rel="stylesheet" type="text/css" href="/DesktopModules/XModPro/styles/rcde-admin/styles.css" />

</ScriptBlock>

<SubmitCommand CommandText="INSERT INTO [RCDEProduct_Categry] (ProductId,CategoryId) VALUES(1,1)" />

<ControlDataSource Id="CategoriesDataSource"

CommandText="

SELECT

CONVERT(VARCHAR,ISNULL(ParentCategoryId,-1))+'|'+CONVERT(VARCHAR,Id) AS [Id]

,'['+ CONVERT(VARCHAR,[CategoryNumber]) + '] '+[Name] AS [Name]

FROM

[RCDECategory]

WHERE

ParentCategoryId IS NOT NULL

ORDER BY

ParentCategoryId,

[Name] ASC"

/>

<ControlDataSource Id="RootCategoriesDataSource"

CommandText="

SELECT

[Id]

,'['+ CONVERT(VARCHAR,[CategoryNumber]) + '] '+[Name] AS [Name]

FROM

[RCDECategory]

WHERE

ParentCategoryId IS NULL

ORDER BY

[Name] ASC"

/>

<div>

<div>

<h3>New Category Assignment for: [[Form:Id]]</h3>

</div>

<div>

<ListBox id="CategoriesListBox" cssclass="collapse"

DataSourceID="CategoriesDataSource" DataTextField="Name" DataValueField="Id" />

<ValidationSummary CssClass="alert alert-danger" Id="ValidationSummary" DisplayMode="List" HeaderText="Please check your entries:"></ValidationSummary>

<div>

<Label For="Category">Category</Label>

<DropDownList id="Category" cssclass="form-control"

DataSourceId="RootCategoriesDataSource" DataValueField="Id" DataTextField="Name" DataType="int32"

Nullable="False" />

<Validate Target="Category" cssclass="text-danger"

Type="required" Text="- Must select a Category." Message="- Must select a Category." />

</div>

<div>

<Label For="Subcategory">Subcategory</Label>

<DropDownList id="Subcategory" cssclass="form-control"

DataType="int32" Nullable="True" />

</div>

</div>

<div>

<AddButton Text="Add" cssclass="btn btn-primary"></AddButton>

<CancelButton Text="Cancel" cssclass="btn btn-default" Visible="true"></CancelButton>

</div>

</div>

<jQueryReady>

//+- Initialize Variabes

var $rootCategories = $('[name$="CategoriesListBox"]');

//+- Define Event Handlers

$('body')

.on('change','select[name$="Category"]',CategoryChange);

//+- Trigger Evens

$('select[name$="Category"]').change();

//+- Event Handlers

function CategoryChange(){

var selectedCategory = $(this).val();

var $options = $rootCategories.find('option[value^="' + selectedCategory + '|"]').clone();

$options.each(function(){

var val = $(this).val();

var categoryId = val.split("|")[1];

$(this).val(categoryId);

});

$('[name$="Subcategory"] option').remove();

$('[name$="Subcategory"]').append($options);

$('[name$="Subcategory"]').closest('.form-group').show();

if($('[name$="Subcategory"] option').length==0){

$('[name$="Subcategory"]').closest('.form-group').hide();

}

}

</jQueryReady>

</AddForm>