Generally speaking, as far as I know, I think Asp.net, for its security, doesn't allow to execute a js script.

However if you really want to do so, please try either of the two ways:

1) 、In web.config, you can see <system.web> tags, please add this inside——<pages validaterequest="false"/>

Sample：

<?xml version="1.0" encoding="gb2312" ?>
<configuration>
<system.web>
<pages validaterequest="false"/>
</system.web>
</configuration>

2、In the head of your aspx page—— 
<%@ page validaterequest="false" language="c#" codebehind="index.aspx.cs" autoeventwireup="false" inherits="mybbs.webform1" %>