Hai u can do like this
<telerik:RadWindow ID="radDelateBMJM" runat="server" VisibleOnPageLoad="false" Height="150px" Title="Message"
Width="250px" Modal="true" Skin="WebBlue" BackColor="#DADADA" VisibleStatusbar="false" Visible="false" Behaviors="Close">
<ContentTemplate>
<table style="width: 100%;">
<tr>
<td></td>
<td></td>
</tr>
<tr>
<td align="right">
<asp:Image ImageUrl="~/Images/Jobs/Help-Support-icon.png" runat="server" />
</td>
<td align="left" class="tdlabelRight">
<asp:Label runat="server" ID="lblDeleteFilter" ForeColor="Black" Text="Do You Want Delete?" Visible="true"></asp:Label>
</td>
</tr>
<tr>
</tr>
<tr>
<td colspan="2" align="center">
<div id="divDelete" runat="server" visible="true">
<telerik:RadButton ID="btnDelete" runat="server" Text="Ok" Skin="Sunset" OnClick="btnDelete_Click"></telerik:RadButton>
<telerik:RadButton ID="rbtnBMCancel" runat="server" Text="Cancel" AutoPostBack="false" Skin="Sunset" OnClientClicked="OnClientClicked"></telerik:RadButton>
</div>
</td>
</tr>
</table>
</ContentTemplate>
</telerik:RadWindow>
in c# ----
on page load
radDelateBMJM.Visible = false;
radDelateBMJM.VisibleOnPageLoad = false;
where u want open like button click there u make it as
radDelateBMJM.Visible = true;
radDelateBMJM.VisibleOnPageLoad = true;