SPOCookie.cs

public static CookieContainer GetO365CookieContainer(SharePointOnlineCredentials credentials, string targetSiteUrl) 
{ 
    Uri targetSite = new Uri(targetSiteUrl); 
    string cookieString = credentials.GetAuthenticationCookie(targetSite); 
    CookieContainer container = new CookieContainer(); 
    string trimmedCookie = cookieString.TrimStart("SPOIDCRL=".ToCharArray());
    container.Add(new Cookie("SPOIDCRL", trimmedCookie, string.Empty, targetSite.Authority)); 
    return container; 
}