I am only guessing as to the exact use you are looking to implement but here is an option if you want to have a form that is not listed in the View menu at all and only appears when needed. First create and name the form you want and then open the blueprint sheet for that form, Select All and copy the contents to the clipboard. Now create a new procedure called .SecretForm and paste the clipboard contents into it. At the very top of this procedure you can add a line like
windowbox "120 70 280 440" (using the dimensions and position you want the form to open at). You can then delete the original form.
When you want that form to appear all you have to do is
call .SecretForm and it will magically open the original form where you want it just as if it always existed. When finished use the
setformoptions with the DELETE option to remove it. You could also go further and copy the contents of that procedure, set a permanent variable like mySecretForm to the clipboard() contents and then remove the procedure and save the file. In the future to open the secret form you would use
On the other hand if you only need a form to open but be invisible you could do as Robert suggested and open it off screen or on screen but too small to be visible using
windowbox "1 1 1 1" which opens it in a one pixel window in the upper left corner of your screen.