添加标头的方法如下:

HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "Your Oauth token");

或者，如果您需要一些自定义标头:

Or if you want some custom header:

HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Add("HEADERNAME", "HEADERVALUE");

此答案已经有SO响应，请参见下文:

This answer has SO responses already, see below:

• 在使用httpClient.GetAsync时添加标头
• 设置HttpClient的授权标头

• Adding headers when using httpClient.GetAsync
• Setting Authorization Header of HttpClient

更新

似乎您要添加两个标题.授权和内容类型.

Seems you are adding two headerrs; authorization and content type.

string authValue = "NLAuth nlauth_account=5731597_SB1,nlauth_email=xxx@xx.com, nlauth_signature=Pswd1234567, nlauth_role=3";
string contentTypeValue = "application/json";

client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(authValue);
client.DefaultRequestHeaders.Add("Content-Type", contentTypeValue);