Postmark Client that supports access to the Administrative APIs, to send email, use the PostmarkClient, instead.
Make sure to include "using PostmarkDotNet;" in your class file, which will include extension methods on the base client.
Inheritance: PostmarkDotNet.PCL.PostmarkClientBase
 protected override async Task SetupAsync()
 {
     _adminClient = new PostmarkAdminClient(WRITE_ACCOUNT_TOKEN);
     var id = Guid.NewGuid();
     _senderprefix = "test-sender-";
     _returnPath = "test." + WRITE_TEST_SENDER_SIGNATURE_PROTOTYPE.Split('@')[1];
     _senderEmail = WRITE_TEST_SENDER_SIGNATURE_PROTOTYPE.Replace("[TOKEN]", String.Format(_senderprefix + "{0:n}", id));
     _replyToAddress = WRITE_TEST_SENDER_SIGNATURE_PROTOTYPE.Replace("[TOKEN]", String.Format(_senderprefix + "replyto-{0:n}@example.com", id));
     _senderName = String.Format("Test Sender {0}", TESTING_DATE);
     await CompletionSource;
 }
        protected override async Task SetupAsync()
        {
            _adminClient = new PostmarkAdminClient(WRITE_ACCOUNT_TOKEN);
            var id = Guid.NewGuid().ToString("n");
            _serverPrefix = "integration-test-server-";

            _name = _serverPrefix + id;
            _color = ServerColors.Purple;
            _rawEmailEnabled = true;
            _smtpActivated = true;
            _inboundHookUrl = "http://www.example.com/inbound/" + id;
            _bounceHookUrl = "http://www.example.com/bounce/" + id;
            _openHookUrl = "http://www.example.com/opened/" + id;
            _postFirstOpenOpenOnly = true;
            _trackOpens = true;
            _inboundSpamThreshold = 30;
            await CompletionSource;
        }