internal static JObject GetToBackOffice(string action)
{
    var url = backOfficeUrl;
    url = url + action;

    HttpClient httpClient = new HttpClient();

    var response =
        httpClient.GetAsync(url
        ).Result;
    if (response.StatusCode == System.Net.HttpStatusCode.OK)
    {
        var idProcess = Newtonsoft.Json.Linq.JObject.Parse(response.Content.ReadAsStringAsync().Result);

        return idProcess;
    }
    else
        return null;

internal static JObject GetToBackOffice(string action)
{
    var url = backOfficeUrl;
    url = url + action;

    HttpClient httpClient = new HttpClient();

    JObject idProcess = new JObject();

    httpClient.GetString(new Uri(url),
        (response) =>
        {
            // Raised when the download completes
            if (response.StatusCode == System.Net.HttpStatusCode.OK)
            {
                idProcess = Newtonsoft.Json.Linq.JObject.Parse(response.Data);
            }
            else
            {
                idProcess = null;
            }
        });
    // Here I would like to wait for the response, so that idProcess is filled with the received data before returning
    return idProcess;
}

IEnumerator WaitForResponce(ResponceType  response)
  {
        while(response.Data == null)
        yield return new WaitForSeconds (0.02f);
   //do what you want here

  }

httpClient.GetString(new Uri(url),
    (response) =>
    {
        // Raised when the download completes
        if (response.StatusCode == System.Net.HttpStatusCode.OK)
        {
            //idProcess = Newtonsoft.Json.Linq.JObject.Parse(response.Data);
            StartCoroutine(WaitForResponce(responce));
        }
        else
        {
            idProcess = null;
        }
    });