How to ? Creating a plugin that open a custom dialog

Hi,

I try to make a plugin that open a dialog only if a condition is true.

I don't understand how to do this (it seems that all plugins using a dialog open it automatically).

My purpose is to create an ajaxSave button that open a dialog, where the user input the name of the file only if it is on creation, before saving.
But if it is an update, the dialog will not be seen by the user.

Thanks for a clue.

Best regards