This is an example asp.net web form project setup to login IdentityServer4 using PKCE flow. The settings to support for login are in web.config.
The project has been pre-configured to login the demo IdentityServer4 at https://cat-token-identity.azurewebsites.net.
If you want to test login with an instance of the IdentityServer4 running on localhost, please read this blog Authenticate Asp.Net Web Form with IdentityServer4. It contains detail instructions to download and run IdentityServer4 on localhost.