HttpClient and Core

I am trying to get an image size from a web site using HttpClient on Core Ver2.0.

I am doing :

private static readonly string browserVer = "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.103 Safari/537.36";
httpClient.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", browserVer);
HttpResponseMessage response = await httpClient.gets(url); 

but when i check the Content-Length header but it not exist.

should i add some settings for the request.