Easiest way to create a custom dialog box which returns a value?

There is no prompt dialog box in C#. You can create a custom prompt box to do this instead.

  public static class Prompt
    {
        public static int ShowDialog(string text, string caption)
        {
            Form prompt = new Form();
            prompt.Width = 500;
            prompt.Height = 100;
            prompt.Text = caption;
            Label textLabel = new Label() { Left = 50, Top=20, Text=text };
            NumericUpDown inputBox = new NumericUpDown () { Left = 50, Top=50, Width=400 };
            Button confirmation = new Button() { Text = "Ok", Left=350, Width=100, Top=70 };
            confirmation.Click += (sender, e) => { prompt.Close(); };
            prompt.Controls.Add(confirmation);
            prompt.Controls.Add(textLabel);
            prompt.Controls.Add(inputBox);
            prompt.ShowDialog();
            return (int)inputBox.Value;
        }
    }

Then call it using:

 int promptValue = Prompt.ShowDialog("Test", "123");

1. On your button set the DialogResult property to DialogResult.OK
2. On your dialog set the AcceptButton property to your button
3. Create a public property in your form called Result of int type
4. Set the value of this property in the click event of your button

5. Call your dialog in this way

   using(myDialog dlg = new myDialog())
   {
       if(dlg.ShowDialog() == DialogResult.OK)
       {
           int result = dlg.Result;
           // whatever you need to do with result
       }
   }