TFS2018 why is shelveset name empty?

When queue-ing a build manually using TFS2018 the shelveset name is not showing the source branch name in all cases. Sometimes it is filled out sometimes not. Since I am picking up build variables for the source branch $(Build.SourceBranch) $(Build.SourceBranchName)

They will be empty if the Shelveset name is empty.

Is it possible to set the shelveset name default as the sourcebranch using the API when creating the build definition?

Is there another build variable that I can use to get the source path for the currently built solution?

UPDATE So I am trying to update the source branches using the build api. However when called I get a

{StatusCode: 405, ReasonPhrase: 'Method Not Allowed', Version: 1.1, Content: System.Net.Http.StreamContent, Headers: { Pragma: no-cache X-TFS-ProcessId: ActivityId: X-TFS-Session: X-VSS-E2EID: X-FRAME-OPTIONS: SAMEORIGIN X-VSS-UserData: :user Persistent-Auth: true Lfs-Authenticate: NTLM X-Content-Type-Options: nosniff Cache-Control: no-cache Date: Fri, 09 Mar 2018 14:37:16 GMT P3P: CP="CAO DSP COR ADMa DEV CONo TELo CUR PSA PSD TAI IVDo OUR SAMi BUS DEM NAV STA UNI COM INT PHY ONL FIN PUR LOC CNT" Server: Microsoft-IIS/10.0 X-AspNet-Version: 4.0.30319 X-Powered-By: ASP.NET Content-Length: 93 Allow: GET Content-Type: application/json; charset=utf-8 Expires: -1 }}

for the following code....

internal void UpdateSourceBranches(List<BuildDefinition> defs)
        {
            using (var handler = new HttpClientHandler { Credentials = new NetworkCredential(tfsUser, tfsPass) })
            using (var client = new HttpClient(handler))
            {
                try
                {
                    client.BaseAddress = new Uri(tfsServer);
                    client.DefaultRequestHeaders.Accept.Clear();
                    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                    foreach (var def in defs)
                    {
                        var buildId = def.Id;
                        var sourceBranch = $"$/{def.Repository.Name}/{def.Project.Name}";
                        var parameters = new Dictionary<string, string> { { "BuildConfiguration", "release" },
                        { "BuildPlatform", "x86|x64|ARM" },
                        { "system.debug", "true" }
                    };

                        var jsonParams = JsonConvert.SerializeObject(parameters);

                        var content = new FormUrlEncodedContent(new[]
                        {
                            new KeyValuePair<string, string>("id", buildId.ToString()),
                            new KeyValuePair<string, string>("sourceBranch", sourceBranch),
                            new KeyValuePair<string, string>("parameters", jsonParams)
                        });

                        var response = client.PostAsync($"DefaultCollection/{def.Repository.Name}/_apis/build/builds?api-version=3.0-preview.1", content);
                        var s = response.Result;
                    }                  
                }
                catch (Exception ex)
                {

                }

            }
        }

You should use Queue a build api to set SourceBranch, for example:

POST http://TFS2018:8080/tfs/DefaultCollection/{project}/_apis/build/builds?api-version=2.0

Content-Type: application/json

{
  "definition": {
    "id": 47
  },
  "sourceBranch":"$/CeceScrum/TestCaseProject",
  "parameters":"{\"BuildConfiguration\":\"release\",\"BuildPlatform\":\"any cpu\",\"system.debug\":\"false\"}"
}

If you want to choose a shelveset to build, then the api looks like below:

POST http://TFS2018:8080/tfs/DefaultCollection/{project}/_apis/build/builds?api-version=2.0

    Content-Type: application/json

    {
      "definition": {
        "id": 47
      },
      "sourceBranch":"ceceShelveset;domain\\username",
      "parameters":"{\"BuildConfiguration\":\"release\",\"BuildPlatform\":\"any cpu\",\"system.debug\":\"false\"}"
    }