Redirecting SubDomain to a SubFolder

I am reading the following article on redirecting sub-domain names to sub folders but where exactly should I put the code:

https://support.discountasp.net/KB/a369/how-to-redirect-a-subdomain-to-a-subdirectory.aspx

Here is my code in Default.aspx.cs file:

protected void Page_Load(object sender, EventArgs e)
{
if((Request.ServerVariables["SERVER_NAME"].ToUpper()).IndexOf(("subdomain.highoncoding.com".ToUpper())) > 0)
{
Response.Redirect("/MySubFolder");
}

}