改写后的代码示例：

var authority = "https://login.microsoftonline.com/{tenantId}";
var clientId = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx";
var clientSecret = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
var scopes = new[] { "api://xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/access_as_user" };

var app = ConfidentialClientApplicationBuilder.Create(clientId)
                .WithClientSecret(clientSecret)
                .WithAuthority(new Uri(authority))
                .Build();

var result = await app.AcquireTokenForClient(scopes)
                .ExecuteAsync();

解决方法说明： 通过ConfidentialClientApplicationBuilder创建ConfidentialClientApplication实例，以获取token。在AcquireTokenForClient方法中传入所需的权限，即scopes。