问题解决方案：在配置B2C策略时，在“Token, claims, and redirect URIs”选项卡中，选择“Access Token”作为返回的令牌类型，并针对“Application ID URI”和“Delegated Permissions”进行相应配置。

代码示例：

public async Task GetAccessTokenAsync()
{
    var accessToken = string.Empty;

    var result = await authenticationContext.AcquireTokenAsync(
        ConfigurationManager.AppSettings["ida:ClientId"],
        ConfigurationManager.AppSettings["ida:RedirectUri"],
        new UserPasswordCredential(ConfigurationManager.AppSettings["userId"], ConfigurationManager.AppSettings["password"]));

    accessToken = result.AccessToken;

    return accessToken;
}