Mini HttpClient json post by Action parameter
Great job. Here are some minor notes though.

In newer versions of C# you can leverage async Main in order not to wait for your test method.

public static async Task Main(string[] args)
{
    await Test();
}


You can speed things up a bit with executing your tests in parallel

public static async Task Test()
{
    var successExecute = Execute("https://jsonplaceholder.typicode.com/posts", new { value = "ITWeiHan" }); //Result : {"value": "ITWeiHan","id": 101}
    var errorExecute = Execute("https://jsonplaceholder.typicode.com/error404", new { value = "ITWeiHan" }); //Result : Error 404
    await Task.WhenAll(successExecute, errorExecute);
}


Also, PostByJsonContentTypeAsync throws couple of more exceptions. Namely InvalidOperationException and ArgumentNullExceptions. Are you intentionally not handling them?