B2C REST API的验证可以通过以下步骤完成：

1. 获取B2C租户的代码和应用程序ID。此ID称为客户端ID。
2. 为B2C REST API创建安全凭据。此凭据称为客户端密钥。
3. 使用客户端ID和客户端密钥创建一个OAuth 2.0授权服务器，并从这个服务器中获取访问令牌。
4. 在REST API的请求中添加访问令牌作为Bearer令牌。

示例代码：

1. 获取B2C客户端ID

2. 获取B2C客户端密钥

3. 创建OAuth 2.0授权服务器并获取访问令牌

string authority = "https://login.microsoftonline.com/[Your-B2C-Tenant-Name].onmicrosoft.com/oauth2/v2.0/token";
string clientId = ConfigurationManager.AppSettings["ida:B2CClientId"];
string clientSecret = ConfigurationManager.AppSettings["ida:B2CClientSecret"];
string scope = "[Your-B2C-REST-API-Scopes]";

var clientCredential = new ClientCredential(clientId, clientSecret);
var authenticationContext = new AuthenticationContext(authority, false);
var result = authenticationContext.AcquireTokenAsync(scope, clientCredential).Result;
string accessToken = result.AccessToken;

4. 在REST API请求中添加访问令牌作为Bearer令牌

HttpClient httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);

string apiUrl = "[Your-B2C-REST-API-URL]";
HttpResponseMessage response = await httpClient.GetAsync(apiUrl);

string apiResponse = await response.Content.ReadAsStringAsync();