Depends on what you want to do, if you want to create your own custom auth provider, inheriting from CredentialsAuthProvider, you can access various request parameters through the IHttpRequest object:
public virtual bool TryAuthenticate(IServiceBase authService, string userName, string password) { var httpReq = authService.RequestContext.Get<IHttpRequest>(); var fromQueryString = httpRequest.QueryString["CustomField"]; var fromPostData = httpRequest.FormData["CustomField"]; var fromEither = httpRequest.GetParam("CustomField");
Here are some other related questions and answers that show the various ways to configure ServiceStack inline Auth:
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