C# How to call a post API by "PostAsJsonAsync" when the specification of header and body of this API is given?

Question

I am learning to use API and I have been given the specification of the URL, header and body of an API. The example is shown as follow

url: api/getRate
hearder:{
  "msgNo": "abc123",
  "receiver": "Customer_A"
}
body:{
  "dateTime": "2023/08/19 18:00:00",
  "Currency": "USD",
  "Ratetype": "COST"
}

How should I use C# method PostAsJsonAsync to get the result by this API?

Currently, I have the following code. Please tell me whether it is a correct way to use PostAsJsonAsync in the aforementioned situation.

var posturl = "api/getRate";
var httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Add("msgNo", "abc123");
httpClient.DefaultRequestHeaders.Add("receiver", "Customer_A");
var requestBody = new {
    "dateTime": "2023/08/19 18:00:00",
    "Currency": "USD",
    "Ratetype": "COST"};
var response = httpClient.PostAsJsonAsync(posturi, requestBody).Result;