Is there a way to disable IIS default authentication in global.asax or somewhere else depending on passed parameters.
A client requests the same form would be secured using IIS authentication and by providing authentication information in POST variables
IIS authentication happens before the request is passed to the ASP.NET egine. Therefore, the answer to your question is ‘no’.
However, by enabling anonymous access on IIS only, you can process any authentication (including Basic etc.) in your application without being ‘intercepted’ by IIS.