UserApiAuthenticationService.cs

public class UserApiAuthenticationService : IUserApiAuthenticationService
{
    private readonly IMemoryCache _memoryCache;
    private readonly HttpClient _httpClient;

    public UserApiAuthenticationService(HttpClient httpClient, IMemoryCache memoryCache)
    {
        _httpClient = httpClient;
        _memoryCache = memoryCache;
    }

    public async Task<string> RetrieveToken()
    {
        DateTime expirationDate;
        if (!_memoryCache.TryGetValue("Token", out string token))
        {
            var body = new { login = "login", password = "password" };
            var response = await _httpClient.PostAsync("login", new StringContent(JsonConvert.SerializeObject(body)));
            (token,  expirationDate) = await response.Content.ReadAsAsync<AuthResponse>();
            _memoryCache.Set("Token", token, new DateTimeOffset(expirationDate));
        }
        return token;
    }
}