A UIWebView subclass for Xamarin iOS that accepts self-signed certificates. Note the caveats at https://ryandavis.io/allowing-uiwebview-to-accept-self-signed-certificates

AcceptSelfSignedCertificateWebView.cs

public class AcceptSelfSignedCertificateWebView : UIWebView
{
    private NSUrlRequest _failedRequest;
    private bool _authenticated;

    private bool OnShouldStartLoad(UIWebView webView, NSUrlRequest request, UIWebViewNavigationType navigationType)
    {
        var result = _authenticated;
        if (!_authenticated)
        {
            _failedRequest = request;
            var urlConnection = NSUrlConnection.FromRequest(request, new AcceptsSelfSignedCertificateUrlConnectionDelegate(this));
            urlConnection.Start();
        }
        return result;
    }

    public AcceptSelfSignedCertificateWebView(RectangleF frame) : base(frame)
    {
        AcceptSelfSignedInit();
    }

    public AcceptSelfSignedCertificateWebView(IntPtr handle) : base(handle)
    {
        AcceptSelfSignedInit();
    }

    public AcceptSelfSignedCertificateWebView()
    {
        AcceptSelfSignedInit();
    }

    private void AcceptSelfSignedInit()
    {
        ShouldStartLoad = OnShouldStartLoad;
    }

    public class AcceptsSelfSignedCertificateUrlConnectionDelegate : NSUrlConnectionDelegate
    {
        private readonly AcceptSelfSignedCertificateWebView _webView;

        private NSUrlRequest _failedRequest => _webView._failedRequest;
        private bool _authenticated { set { _webView._authenticated = value; } }

        public AcceptsSelfSignedCertificateUrlConnectionDelegate(AcceptSelfSignedCertificateWebView parent)
        {
            _webView = parent;
        }

        public override void WillSendRequestForAuthenticationChallenge(NSUrlConnection connection,
            NSUrlAuthenticationChallenge challenge)
        {
            if (challenge.ProtectionSpace.AuthenticationMethod == NSUrlProtectionSpace.AuthenticationMethodServerTrust)
            {
                var challengeHost = challenge.ProtectionSpace.Host;
                var baseUrlHost = _failedRequest.Url.Host;

                if (challengeHost == baseUrlHost)
                    challenge.Sender.UseCredentials(NSUrlCredential.FromTrust(challenge.ProtectionSpace.ServerSecTrust), challenge);
            }

            challenge.Sender.ContinueWithoutCredentialForAuthenticationChallenge(challenge);
        }

        public override void ReceivedResponse(NSUrlConnection connection, NSUrlResponse response)
        {
            _authenticated = true;
            connection.Cancel();

            _webView.LoadRequest(_failedRequest);
        }
    }
}