可參考：
http://stackoverflow.com/questions/12373738/how-do-i-set-a-cookie-on-httpclients-httprequestmessage/13287224#13287224
https://d-fens.ch/2016/12/27/howto-set-cookie-header-on-defaultrequestheaders-of-httpclient/
重點兩種
【預設：HttpClientHandler(UseCookies=true)】
HttpClient <--httpclienthandler ookiecontainer="" p="">
cookieContainer.Add(baseAddress, new Cookie("CookieName", "cookie_value"));
【HttpClient <--httpclienthandler p="">
若是：HttpClientHandler(UseCookies=false) 則是手動加入header
httpClient.DefaultRequestHeaders.Add("Cookie", "auth=ArbitrarySessionToken");