GET https://{account}.visualstudio.com/{project}/_apis/wit/workitems/{id}?$expand=Relations&api-version=5.0-preview.3

using System;
using Microsoft.VisualStudio.Services.Client;
using Microsoft.VisualStudio.Services.WebApi;
using Microsoft.TeamFoundation.WorkItemTracking.WebApi;
using Microsoft.TeamFoundation.WorkItemTracking.WebApi.Models;

namespace GetWorkItemFullyExpanded
{
    class Program
    {
        static void Main(string[] args)
        {
            GetWorkItemFullyExpanded();
        }

        public static WorkItem GetWorkItemFullyExpanded()
        {
            int id = xx;

            var myCredentials = new VssClientCredentials();
            var connection = new VssConnection(new Uri(@"https://xxxx.visualstudio.com"), myCredentials);
            WorkItemTrackingHttpClient workItemTrackingClient = connection.GetClient<WorkItemTrackingHttpClient>();

            WorkItem workitem = workItemTrackingClient.GetWorkItemAsync(id, expand: WorkItemExpand.Relations).Result;

            Console.WriteLine(workitem.Id);

            Console.WriteLine("Relations: ");
            foreach (var relation in workitem.Relations)
            {

                if (relation.Rel == "Microsoft.VSTS.Common.TestedBy-Forward")
                    Console.WriteLine(relation.Url);

            }

            return workitem;
        }

    }
}