Do it in the Server code: the C#, not HTML / Javascript.
User logs in, his username and password are passed to the server.
The server validates the username / password combination (using a salted and hashed password stored against the username in the DB) and if valid retrieves the user type at the same time from the same DB: Admin or Member.
If it's a member, you redirect him to the generic "success page", it's it's an admin, you redirect to an Admin menu:
HttpResponse.Redirect Method (System.Web) | Microsoft Learn[
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