Simple do this:
if(username == "Admin" && password == "Admin") {
Response.Redirect("~/new/page");
}
But do you really want to do that? No! That's exactly why there are authentication systems provided. In ASP.NET, you are provided with Identity system that can be used to provide a great and secure authentication system.
I don't find any reason to develop an authentication system from scratch. Also, if you really want to, please hash the password before saving. It is not good at all, the way it is. Any potential user can read the file and know what password Admin has.
Also, since Identity supports Roles. You would create users in Admin role and change the UI for the application for them. Otherwise, provide a basic one. I don't want to help you build a new one, instead, just learn how to do that in ASP.NET and allow it to maintain the authentication for you;
they have a great authentication service and have tested in many cases.
ASP.NET Identity | The ASP.NET Site[
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