<endpoint address="http://tempuri.org/myExternalServiceEndPointAddress"
    binding="basicHttpBinding" bindingConfiguration="myExternalServiceBinding" contract="myExternalServiceContract" name="MyExternalServiceName">
    <headers>
      <wsse:Security xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
        <wsse:UsernameToken>
          <wsse:Username>myUsername</wsse:Username>
          <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">myPassword</wsse:Password>
        </wsse:UsernameToken>
      </wsse:Security>
    </headers>
  </endpoint>

myserviceClientObject.Endpoint.Address = new EndpointAddress(newEndpointUrl);

myServiceClientObject.ClientCredentials.Username.Username = myUsername;
myServiceClientObject.ClientCredentials.Username.Password = myPassword;