Dim strAdress As String = "https://prozorro.gov.ua/tender/search?edrpou=20915546"
        Dim web As New WebClient
        web.Encoding = System.Text.Encoding.UTF8
        Dim strQlltext As String = web.DownloadString(strAdress)
        RichTextBox1.Text = strQlltext

ServicePointManager.ServerCertificateValidationCallback += (sender, cert, chain, sslPolicyErrors) => true;

 Try
            Dim strAdress As String = "https://prozorro.gov.ua/tender/search?edrpou=20915546"
            Dim web As New WebClient
            web.Encoding = System.Text.Encoding.UTF8
            Dim strQlltext As String = web.DownloadString(strAdress)
        
             System.Net.ServicePointManager.ServerCertificateValidationCallback = _
            Function(se As Object, _
            cert As System.Security.Cryptography.X509Certificates.X509Certificate, _
            chain As System.Security.Cryptography.X509Certificates.X509Chain, _
            sslerror As System.Net.Security.SslPolicyErrors) True
            RichTextBox1.Text = strQlltext
        
            ' GBA(strQlltext, "<li class=""marked"">", "</li>", ListBox1)
        Catch ex As Exception
            MsgBox(ex.ToString)

        End Try

   Dim strQlltext As String = web.DownloadString("https://prozorro.gov.ua/tender/search?edrpou=20915546")

using (WebClient wc = new WebClient()) {
     //wc.Headers.Add("User-Agent", "DotNET WebClient");
     //htmlsrc = wc.DownloadString(pageuri);
     var rawdata = wc.DownloadData(pageuri);
     htmlsrc = Encoding.UTF8.GetString(rawdata);
}

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12