Use token method GET WebAPI HttpClient

Samuel is completely right. To give you more information, understand that:

client.DefaultRequestHeaders.Authorization

is equivalent to:

HTTP/1.1
Authorization: <tipo> <token>

DefaultRequestHeaders is a collection of keys and values, equal to Dictionary<TKey,TValue> . So, the following is exactly the same as the previous one:

client.DefaultRequestHeaders.Add("Authorization", "<tipo> <token>");

If you want something like this:

HTTP/1.1
X-Auth-Token: <qualquer coisa>

I would have to use the same as @SamuelMcAravey said, which would be:

client.DefaultRequestHeaders.Add("X-Auth-Token", "<qualquer coisa>");

More information about HTTP headers can be found in various sources on the internet, this article from Wikipedia being one of them.

The problem is here:

client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("X-Auth-Token", vAuth_token);

What happens is that the HTTP header looks something like this:

Authorization: X-Auth-Token <<seu token aqui>>

When you are building the AuthenticationHeaderValue the first parameter is called scheme. You see this used enough for Bearer Authentication, but I think you're just wanting it to be a normal header. To do this you can try something like this:

client.DefaultRequestHeaders.Add("X-Auth-Token", vAuth_token);

And then you will see a header so it should work:

X-Auth-Token: <<seu token>>

I needed to first call the login page with the Token to login, and then call the ADMIN method.

Resolution:

if (retorno.valid == "true") {
response = null;
client = null;
vAuth_token = retorno.authentication_token;

client = new HttpClient(httpClientHandler);
client.BaseAddress = new Uri("http://xxxxxx.xxxxxx.xxx/");
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
response = await client.PostAsJsonAsync("api/login?auth_token=" + vAuth_token, "");

response = null;
client = null;
client = new HttpClient(httpClientHandler);
client.BaseAddress = new Uri("http://xxxxxx.xxxxxx.xxx/");
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
client.DefaultRequestHeaders.Add("X-Auth-Token", vAuth_token);
response = await client.GetAsync("api/admin/users");
if (response.IsSuccessStatusCode) {
    // HTTP GET
    try
    {
        //Usuarios usuarios = await response.Content.ReadAsAsync<Usuarios>();
        var data = response.Content.ReadAsStringAsync().Result;
    }
    catch (Exception erro)
    {

    }
}

Thank you!