<form method="post">
    <button type="submit" asp-page-handler="down" class="btn btn-primary">短信发送</button>
   
</form>

 public async Task<IActionResult> OnPostDown()
        {           

            string url = "https://smssh1.253.com/msg/v1/send/json";

            var response = await url.SetBody(new Dictionary<string, object> { { "account", "" }, { "password", "" }, { "msg", "123456" }, { "phone", "18072707664" } }, "application/json", Encoding.UTF8).PostAsStringAsync();


            return Content(response);

        }