Every Radzen Blazor application with Security enabled has the following code:
var query = System.Web.HttpUtility.ParseQueryString(new Uri(UriHelper.ToAbsoluteUri(UriHelper.Uri).ToString()).Query);
var error = query.Get("error");
redirectUrl = query.Get("redirectUrl");;
That's how to work with query string parameters. Those are all Execute C# actions.
Can you point me to such questions so I can link my reply in this thread?