How to open form as dialog in Slack app with Slash Command and submit the form data to server using ASP.NET MVC

Question

How to open form as dialog in Slack app with Slash Command and submit the form data to server using ASP.NET MVC.

I have created a controller in ASP.NET MVC and created these methods.

First method is to open the form and second method will be called after form submission.

public ActionResult DisplayForm()
{ 
    return View();
}
    
[HttpPost]
public ActionResult DisplayForm(FormCollection fc)
{
    return Content("Thanks for submitting the form!");
}

In Slack, after doing all set up I have created the slash command named as /testleave but on firing the command it is generating error dispatch_failed error.

/testleave failed with the error "dispatch_failed" == error in slack app