Run QueuePublish and QueueCheckIn Method from workflow using ASMX service - authentication error

Hi

I need to update custom field from workflow so using UpdateProjectProperty which needs QueuePublish and QueueCheckIn Methods to be called from workflow for updation.

I have add web reference of ProjectServerServices.dll into my solution and added following code for authentication before QueuePublish

project.UseDefaultCredentials = true;
project.Credentials = System.Net.CredentialCache.DefaultCredentials;
project.QueuePublish(jobId, projectuid, true, String.Empty);

I am getting error as

The request failed with HTTP status 401: Unauthorized.

Can somebody help me out with some sample code for authentication needed for using PSI with workflow in asmx service?