how to serialize a json string to a form post data

This can be done by first deserializing your JSON to a Dictionary<string, string>, then iterating through the key-value pairs in the dictionary and building up a querystring from that.

However, keep in mind that querystring format (application/x-www-form-urlencoded) is not a hierarchical format, while JSON is. So your JSON object can only be a simple object with key-value pairs (no arrays or nested objects). If your JSON is more complicated than that, you will have to do some more work to flatten it before you can convert it to a querystring.

Demo:

class Program
{
    static void Main(string[] args)
    {
        string json = @"
        {
            ""key1"" : ""value1"",
            ""key2"" : ""value2"",
            ""int"" : 5,
            ""bool"" : true,
            ""decimal"" : 3.14,
            ""punct"" : ""x+y=z""
        }";

        var dict = JsonConvert.DeserializeObject<Dictionary<string, string>>(json);

        StringBuilder sb = new StringBuilder();
        foreach (KeyValuePair<string, string> kvp in dict)
        {
            if (!string.IsNullOrEmpty(kvp.Key) && !string.IsNullOrEmpty(kvp.Value))
            {
                if (sb.Length > 0) sb.Append('&');
                sb.Append(HttpUtility.UrlEncode(kvp.Key));
                sb.Append('=');
                sb.Append(HttpUtility.UrlEncode(kvp.Value));
            }
        }
        var postDataString = sb.ToString();

        Console.WriteLine(postDataString);
    }
}

Output:

key1=value1&key2=value2&int=5&bool=True&decimal=3.14&punct=x%2by%3dz

As was mentioned in the comments, you can use the FormUrlEncodedContent class to do the same thing. Replace the StringBuilder and foreach loop in the code above with the following (but note this approach requires async/await):

var formUrlEncodedContent = new FormUrlEncodedContent(dict);
var postDataString = await formUrlEncodedContent.ReadAsStringAsync();