Pass Windows User (domain\userid) to report url in ASCX page

Good day, This is linked to my previous post.

I want to display the Windows user logged on to the PC (domain\userid) and add to the report URL.

I have searched:

Code:

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<% Response.Write(Request.ServerVariables("AUTH_user")) %>

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Now i want to display & add to url:

Code:

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<%@ Control Language="c#" AutoEventWireup="True" Codebehind="Header.ascx.cs" Inherits="FlowCentric.Net.Navigator.Header" TargetSchema="http://schemas.microsoft.com/intellisense/ie5"%>
<table><tr>
<td><asp:Label id="lblLogin" runat="server">We</asp:Label></td>             
</tr></table>

 <IFRAME id="frame1" src="http://biweb/reportserver?&User= <% Request.ServerVariables("AUTH_user") %>"></IFRAME>       
 
<% Response.Write(Request.ServerVariables("LOGon_user")) %>  <!-- this does not work

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Please Assist!