UserServiceWithPrivateMethod.cs

public class UserService : IUserService
{
    private readonly HttpClient _httpClient;
    public UserService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task<IReadOnlyCollection<User>> GetAllUsers()
    {
        var token = await RetrieveToken();
        var request = new HttpRequestMessage(HttpMethod.Get, "user");
        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
        var response = await _httpClient.SendAsync(request);
        response.EnsureSuccessStatusCode();
        return await response.Content.ReadAsAsync<IReadOnlyCollection<User>>();
    }

    public async Task UpdateUser(User userToUpdate)
    {
        var token = await RetrieveToken();
        var request = new HttpRequestMessage(HttpMethod.Put, $"user/{userToUpdate.Name}");
        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
        request.Content = new StringContent(JsonConvert.SerializeObject(userToUpdate));
        var response = await _httpClient.SendAsync(request);
        response.EnsureSuccessStatusCode();
    }

    private async Task<string> RetrieveToken()
    {
        var body = new { login = "login", password = "password" };
        var response = await _httpClient.PostAsync("login", new StringContent(JsonConvert.SerializeObject(body)));
        var authResponse = await response.Content.ReadAsAsync<AuthResponse>();
        return authResponse.Token;
    }
}