So I saw one of the old thread by Princy:
https://www.telerik.com/forums/find-control-within-itemtemplate-client-side
I tried that in my lightbox but it seems is not setting the value of the textbox in the itemtemplate of the lightbox, why ?
function OpenLB() {
var lightBox = $find('<%= LBoxEdit.ClientID %>');
lightBox.show();
var Text = lightBox.get_element().getElementsByClassName("rltbDescriptionBox");
Text.LBoxEdit$txtDescripton.control.set_value("new value");
}
<telerik:RadLightBoxItem>
<ItemTemplate>
<table style="padding: 20px; font-family: Arial">
<tr>
<td style="font-family: Arial, Helvetica, sans-serif; font-size: 18px; padding-bottom: 10px;">
Description
</td>
</tr>
<tr>
<td style="padding: 10px 0px 10px 20px">
<telerik:RadTextBox ID="txtDescripton" runat="server" CssClass="rltbDescriptionBox" Font-Size="16px" Width="400px" Height="30px">
</telerik:RadTextBox>
</td>
</tr>
<tr>
<td style="padding: 10px 0px 10px 20px">
<telerik:RadButton RenderMode="Lightweight" ID="btnFormSave" Height="30px"
Text="Save" Font-Size="16px" Font-Bold="True" runat="server" Skin="Silk"></telerik:RadButton>
</td>
</tr>
</table>
</ItemTemplate>
</telerik:RadLightBoxItem>