Just create another form (let’s call it formPopup) using Visual Studio. In a button handler write the following code:
var formPopup = new Form();
formPopup.Show(this); // if you need non-modal window
If you need a non-modal window use: formPopup.Show();. If you need a dialog (so your code will hang on this invocation until you close the opened form) use: formPopup.ShowDialog()