Convert JSON string to C# class in ASP.Net

I want to pass nested parameters with HTTP client POST. How can I achieve this I have the following code:

string credentials = JsonConvert.SerializeObject(new
{
    ApiKey= "123456",
});
HttpClient client = new HttpClient();
client.BaseAddress = new Uri("https://test/tokens");
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));//ACCEPT header
HttpRequestMessage request = new HttpRequestMessage();
request.Method = HttpMethod.Post;
request.Content = new StringContent(credentials, Encoding.UTF8, "application/json");

My JSON file looks like this. How can I pass all these nested parameters with HTTP client post.

{
    "OrderTotal":null,
    "MerchantCode":"Test1",
    "MerchantKey":"Test2",
    "ServiceCode":"Testservice1",
    
 
    "Phone":"123-456-7899",
    "Email":"test@test.com",
    "CustomerId" : null,
    "CompanyName" : "",
    "CustomerAddress":{
        "Name":"Thomas jefferson",
        "Address1":"123 test drive",
        "Address2":"",
        "City":"LCK",
        "State":"MI",
        "Zip":"12345",
        "Country":"US"
    },
    "LineItems":
    [
        {
        "Sku":"TEST",
        "Description":"Test",
        "UnitPrice":100.00,
        "Quantity":1
        }
       
    ]
}

I defined these two classes too:

public class MainParametrs
{
        public string MerchantCode { get; set; }
        public string MerchantKey { get; set; }
        public string ServiceCode { get; set; }
        public string UniqueTransId { get; set; }
        public string LocalRef { get; set; }
        public string SuccessUrl { get; set; }
        public string FailureUrl { get; set; }
        public string DuplicateUrl { get; set; }
        public string CancelUrl { get; set; }
        public string Phone { get; set; }
        public string CustomerAddress { get; set; }
        public string LineItems { get; set; }
}
public class CustomerAddressParam
{
        public string Name { get; set; }
        public string Address1 { get; set; }
        public string Address2 { get; set; }
        public string City { get; set; }
        public string State { get; set; }
        public string Zip { get; set; }
        public string Country { get; set; }
}

Any help will be highly appreciated.