client2.cs

using (var client = new HttpClient())
{
    client.BaseAddress = new System.Uri("https://<your tenant>.auth0.com/");
    var response = await client.PostAsync("oauth/token", new FormUrlEncodedContent(
        new Dictionary<string, string>
        {
                            { "grant_type", "client_credentials" },
                            { "client_id", "<client_id>" },
                            { "client_secret", "<client_secret>" },
                            { "audience", "https://<your tenant>.auth0.com/api/v2/" }
        }
    ));
    
    var content = await response.Content.ReadAsStringAsync();
    var jsonResult = JObject.Parse(content);
    
    var mgmtToken = jsonResult["access_token"].Value<string>();
    using (var mgmtClient = new ManagementApiClient(mgmtToken, new System.Uri("https://<your tenant>.auth0.com/api/v2")))
    {
        return await mgmtClient.Users.GetAsync(userId);
    }
}