How to fire Required field validation when clicking Add New Item in Rad Grid

If (TypeOf e.Item Is GridEditableItem AndAlso e.Item.IsInEditMode) Then

            Dim editedItem As GridEditableItem = CType(e.Item, GridEditableItem)

            Dim requiredFieldRemark As RequiredFieldValidator = DirectCast(editedItem.FindControl("requiredFieldRemark"), RequiredFieldValidator)
            requiredFieldRemark.Enabled = True

ElseIf (TypeOf e.Item Is GridEditFormInsertItem AndAlso e.Item.OwnerTableView.IsItemInserted) Then

            Dim dataItem As GridEditableItem = CType(e.Item, GridEditableItem)
            Dim requiredFieldRemark As RequiredFieldValidator = DirectCast(dataItem.FindControl("requiredFieldRemark"), RequiredFieldValidator)
            requiredFieldRemark.Enabled = True
 End If

Remarks:
<telerik:radgrid id="rgRemarks" runat="server" allowpaging="True" allowsorting="True"
="" autogeneratecolumns="False" gridlines="None" allowmultirowedit="false" allowautomaticinserts="false" allowautomaticupdates="false" cellspacing="0" width="500px" style="float: right">

<pagerstyle mode="NextPrevAndNumeric" alwaysvisible="True">
<mastertableview commanditemdisplay="Top" datakeynames="EmployeeID, RemarkID" cellspacing="-1" retrievealldatafields="false">
<commanditemsettings showrefreshbutton="True" showaddnewrecordbutton="True" addnewrecordtext="Add Remark">
<columns>
<telerik:gridtemplatecolumn datafield="Remark" sortexpression="Remark" headertext="Remark"
="" uniquename="Remark">
<itemtemplate>
<asp:label id="lblRemark" text="<%#Eval("Remark")%>" runat="server">


<edititemtemplate>
<asp:textbox id="txtRemark" runat="server" textmode="MultiLine" width="300px">
<asp:requiredfieldvalidator id="requiredFieldRemark" runat="server" controltovalidate="txtRemark"
="" errormessage="* Remark is required field." display="Dynamic">




<telerik:gridbuttoncolumn buttontype="ImageButton" headertext="Edit" commandname="Edit" text="Edit">
<telerik:gridbuttoncolumn confirmtext="Delete this Remark?" confirmdialogtype="RadWindow"
="" confirmtitle="Delete" buttontype="ImageButton" commandname="Delete" text="Delete" headertext="Delete" uniquename="DeleteColumn">


<editformsettings>
<editcolumn buttontype="ImageButton" cancelimageurl="~\Images\CommandImages\Cancel.gif" canceltext="Cancel Edit"
="" insertimageurl="~\Images\CommandImages\Save.gif" inserttext="Insert" uniquename="EditColumn" updateimageurl="~\Images\CommandImages\Save.gif" updatetext="Update ">