plugin page – but not on the menu

This may be a very silly question, but I have searched and cannot find an answer. In fact I am sure that it will be a doh! moment when I do get an answer.

I am currently coding a plugin, and have been asked to provide an email form that is accessed *only* from a certain admin page.

If it was to be a menu item I can add that and access it without a problem. But I just can’t figure out how to access a plugin page that isn’t linked from the menu!

Can someone point me in the right direction please.