Popup menu button - how to prepare the button?

[Keith Clarke]

Hi folks,
I'm trying to set a contextual popup menu to fire when right-clicking in a table field - in any 'cell' - but can't find any documentation on how to prepare the button itself (so that it's not visible but then appears at the mouse location).

My script on the table field is:

on mouseDown
   if theButton is 3 then popup menu RowMenu
   else pass mouseDown
end mouseDown

…meanwhile, the RowMenu button just sits happily in the location I created it.

Lessons and docs seem to gloss over the button end of the recipe - so what's the secret, does it need to be on a different card or sub-stack?
Best, 
Keith..