How to consume other API into our Webapi

Question

Sorry , I am new in web api actually when I am running my API on postman the response is correct.but when i am implementing in code its giving internal server code error.

var gizmo = new Product() { ChannelId= "948404"};
HttpResponseMessage response = await client.PostAsJsonAsync("https://xxx-xxxxxx-api.xxx.co/api/bills, , gizmo);
if (response.IsSuccessStatusCode)
{
}