HttpClient not supporting PostAsJsonAsync method C#

Question

I am trying to call a web API from my web application. I am using .Net 4.5 and while writing the code I am getting the error HttpClient does not contain a defin

Answer 1

Just expanding Jeroen's answer with the tips in comments:

var content = new StringContent(
    JsonConvert.SerializeObject(user), 
    Encoding.UTF8, 
    MediaTypeNames.Application.Json);

var response = await client.PostAsync("api/AgentCollection", content);