Transfer from .NET Core 3.1 to .NET 6.0 error

I changed from .NET Core 3.1 to 6.0, and now I am getting a squiggly error under DownloadString. I believe it is to do with HttpClient what using directive or assembly reference should I be using?

I am already using System.Net.Http.

Error CS1061
'HttpClient' does not contain a definition for 'DownloadString' and no accessible extension method 'DownloadString' accepting a first argument of type 'HttpClient' could be found (are you missing a using directive or an assembly reference?

var gReply = client.DownloadString(string.Format(
                        "https://www.google.com/recaptcha/api/siteverify?secret={0}&response={1}",
                        secretKey, gResponse));