I got the solution:
ServicePointManager.ServerCertificateValidationCallback =
(object s, X509Certificate certificate, X509Chain chain,
SslPolicyErrors sslPolicyErrors) => true;
SecurityBindingElement securityElement = SecurityBindingElement.CreateUserNameOverTransportBindingElement();
securityElement.IncludeTimestamp = false;
TextMessageEncodingBindingElement encodingElement = new TextMessageEncodingBindingElement(MessageVersion.Soap11, Encoding.UTF8);
HttpsTransportBindingElement transportElement = new HttpsTransportBindingElement();
CustomBinding customBinding = new CustomBinding(securityElement, encodingElement, transportElement);
EndpointAddress endpoint = new EndpointAddress("https://10.235.135.219:8443/soap/efecty/");
ServicioPlaceToPay.PlacetoPay_EfectyPortClient client = new PlacetoPay_EfectyPortClient(customBinding, endpoint);
client.ClientCredentials.UserName.UserName = "juan";
client.ClientCredentials.UserName.Password = "j321*";
ServicioPlaceToPay.BillResponse a = client.getBillByReference("1248");