C# HttpClient Usert Agent headers are not adding

i came across strange problem. I just set User agent headers in such way:

var httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Add("Client", "User-Agent:Chrome/65.0.3325.181");

var response = await httpClient.GetStringAsync(url);

But when i check it in fiddler i don't see such header. Maybe someone can explain to me what i do wrong?