Since DotNetBrowser 1.11 it is possible to select SSL certificate instantiated from standard X509Certificate class. For example:

C#

using DotNetBrowser;
using DotNetBrowser.WinForms;
using System;
using System.IO;
using System.Security.Cryptography.X509Certificates;
using System.Windows.Forms;

namespace TestWinFormsCustomCertificate
{
    public partial class Form1 : Form
    {
        WinFormsBrowserView browserView;

        public Form1()
        {
            InitializeComponent();
        
            browserView = new WinFormsBrowserView();
            Controls.Add((Control)browserView);
            Browser browser = browserView.Browser;

            X509Certificate x509Certificate = new X509Certificate(
                Path.GetFullPath("client.pfx"), 
                "1234", X509KeyStorageFlags.Exportable);
            Certificate certificate = new Certificate(x509Certificate);
            CertificatesDialogParams dialogParams = null;

            browser.DialogHandler = new SelectCertificateDialogHandler(
                this, 
                (selectCertificateParameters) =>
            {
                dialogParams = selectCertificateParameters;
                dialogParams.SelectedCertificate = certificate;
                return CloseStatus.OK;
            });
            browser.LoadURL("https://ssl-host:4443");
        }

        class SelectCertificateDialogHandler : WinFormsDefaultDialogHandler
        {
            private Func<CertificatesDialogParams, CloseStatus> certificateAction;

            public SelectCertificateDialogHandler(Control control,
                Func<CertificatesDialogParams, CloseStatus> certificateAction)
                :base(control)
            {
                this.certificateAction = certificateAction;
            }

            public override CloseStatus OnSelectCertificate(
                CertificatesDialogParams parameters)
            {
                return certificateAction(parameters);
            }
        };
    }
}

VB.NET

Imports System.IO
Imports System.Security.Cryptography.X509Certificates
Imports DotNetBrowser
Imports DotNetBrowser.WinForms


Public Partial Class Form1
    Inherits Form

    Private browserView As WinFormsBrowserView

    Sub New()
        InitializeComponent()
        browserView = New WinFormsBrowserView()
        Controls.Add(CType(browserView, Control))
        Dim browser As Browser = browserView.Browser
        Dim x509Certificate As X509Certificate = New X509Certificate(Path.GetFullPath("client.pfx"), "1234",
                                                                     X509KeyStorageFlags.Exportable)
        Dim certificate As Certificate = New Certificate(x509Certificate)
        Dim dialogParams As CertificatesDialogParams = Nothing
        browser.DialogHandler = New SelectCertificateDialogHandler(Me, Function(selectCertificateParameters)
            dialogParams = selectCertificateParameters
            dialogParams.SelectedCertificate = certificate
            Return CloseStatus.OK
        End Function)
        browser.LoadURL("https://ssl-host:4443")
    End Sub

    Class SelectCertificateDialogHandler
        Inherits WinFormsDefaultDialogHandler

        Private certificateAction As Func(Of CertificatesDialogParams, CloseStatus)

        Public Sub New(ByVal control As Control,
                       ByVal certificateAction As Func(Of CertificatesDialogParams, CloseStatus))
            MyBase.New(control)
            Me.certificateAction = certificateAction
        End Sub

        Public Overrides Function OnSelectCertificate(ByVal parameters As CertificatesDialogParams) As CloseStatus
            Return certificateAction(parameters)
        End Function
    End Class
End Class