sample refactoring

main.cs

    public interface IApiClient
    {
        Task<T> Call<T>(string uri, object request, CancellationToken cancellationToken);
        Task<string> GetToken();
    }
    public class ApiClient: IApiClient
    {
        protected ApiClient()
        {
            Client = new HttpClient();
        }

        private HttpClient Client { get; }
        private const string BaseUrl = "https://server.tld";
        private const string ServiceAccountUserName = "user";
        private const string ServiceAccountUserPassword = "password";

        public async Task<T> Call<T>(string uri, object request, CancellationToken cancellationToken)
        {
            var token = await GetToken();

            var result = await Post<T>(uri, request, token);

            return result;
        }

        private async Task<TResponse> Post<TResponse>(string uri, object request, string token = null)
        {
            var jsonString = JsonConvert.SerializeObject(request);
            var content = new StringContent(jsonString, Encoding.UTF8, "application/json");
            if (!string.IsNullOrWhiteSpace(token))
            {
                Client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
            }
            var response = await Client.PostAsync(uri, content);

            var result = await response.Content.ReadAsStringAsync();
            var deserialized = JsonConvert.DeserializeObject<TResponse>(result);
            return deserialized;
        }

        public async Task<string> GetToken()
        {
            var request = new TokenRequest
            {
                UserName = ServiceAccountUserName,
                Password = ServiceAccountUserPassword,
            };
            const string url = "/api/token";
            var answer = await Post<TokenAnwer>(url, request);
            return answer.Token;
        }

        private class TokenRequest
        {
            public string UserName { get; set; }
            public string Password { get; set; }
        }

        public class TokenAnwer
        {
            public string Token { get; set; }
            //public string UserName { get; set; }
        }
    }
    public class CacheTokenApiClient : IApiClient
    {
        private readonly ApiClient apiClient;

        public CacheTokenApiClient(ApiClient apiClient)
        {
            this.apiClient = apiClient;
        }

        public Task<T> Call<T>(string uri, object request, CancellationToken cancellationToken)
        {
            return apiClient.Call<T>(uri, request, cancellationToken);
        }

        private string token;
        public async Task<string> GetToken()
        {
            if (!string.IsNullOrEmpty(token))
            {
                return token;
            }

            token = await apiClient.GetToken();
            return token;
        }
    }