Onde posso definir o cabeçalho como chamada de serviço REST ao usar HTTPClient simples?

Eu faço :

    HttpClient client = new HttpClient();
    var values = new Dictionary<string, string>
{
        {"id", "111"},
        {"amount", "22"}
};
    var content = new FormUrlEncodedContent(values);
    var uri = new Uri(@"https://some.ns.restlet.uri");

    var response = await client.PostAsync(uri, content);
    var responseString = await response.Content.ReadAsStringAsync();

UPD

O cabeçalho que eu não adiciono é:

{"Authorization":"NLAuth nlauth_account=5731597_SB1, [email protected], nlauth_signature=Pswd1234567, nlauth_role=3","Content-Type":"application/json"}

Devo seguir:

client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Authorization", "NLAuth nlauth_account=5731597_SB1, [email protected], nlauth_signature=Pswd1234567, nlauth_role=3","Content-Type":"application/json");