How to trigger a build in TFS 2015 using REST API

This works like a charm without REST

var tfsurl = new Uri("http://localhost:8080/tfs/<***projectname***>/");    
var ttpc = new TfsTeamProjectCollection(tfsurl);
var bhc = ttpc.GetClient<BuildHttpClient>();
var builds = bhc.GetBuildsAsync("<***projectname***>").Result;
var build = builds
    .Where(x => x != null && x.Definition.Name.Equals("***buildDefinitionName***>"))
    .OrderByDescending(y => y.LastChangedDate)
    .FirstOrDefault();

bhc.QueueBuildAsync(build);

TFS 2015 RC2 uses a new API (version 2.0-preview.2). The VSO sample I mentioned in the question is outdated and not relevant when you wish to queue a new build.

Currently, there is no documentation but the web portal uses REST API so just Fiddler away.

Here is the code:

var buildRequestPOSTData =
                    new BuildRequest()
                    {
                        Definition = new Definition()
                        {
                            Id = firstBuildDefinition.Id
                        },
                        Project = new Project { Id = "project guid" },
                        Queue = new Queue {  Id = 1 },
                        Reason = 1,
                        sourceBranch = "$Branch"
                    };

                responseBody = await QueueBuildAsync(client, buildRequestPOSTData, _baseUrl + "build/Builds");

And here is the class with new parameters for build requests:

public class BuildRequest
{
    [JsonProperty(PropertyName = "definition")]
    public Definition Definition { get; set; }

    [JsonProperty(PropertyName = "demands")]
    public string Demands { get; set; }

    [JsonProperty(PropertyName = "parameters")]
    public IEnumerable<string> Parameters { get; set; }

    [JsonProperty(PropertyName = "project")]
    public Project Project { get; set; }

    [JsonProperty(PropertyName = "queue")]
    public Queue Queue { get; set; }

    [JsonProperty(PropertyName = "reason")]
    public int Reason { get; set; }

    [JsonProperty(PropertyName = "sourceBranch")]
    public string sourceBranch { get; set; }

    [JsonProperty(PropertyName = "sourceVersion")]
    public string RequestedBy { get; set; }
}

public class Definition
{
    [JsonProperty(PropertyName = "id")]
    public int Id { get; set; }
}

public class Queue
{
    [JsonProperty(PropertyName = "id")]
    public int Id { get; set; }
}

public class Project
{
    [JsonProperty(PropertyName = "id")]
    public string Id { get; set; }
}