Details Required entry fields not filled; click here to continue anyway

Hi.

I have Z AB with mandatory fields in a BP.

I validate this fields in EH_ONSAVE event and I'm showing an error message if it not filled.

But, with mandatory fields empties, if I clicked, i.e., to 'Business Partner' button to exit of the BP, in the screen is showing this message "DetailsRequired entry fields not filled; click here to continue anyway".

If I clicked to the option, popup is displayed with "Do you want to save your changes?", and if I select YES, partner is saved even with mandatory fields empies.

I don't know where is the event to this popup.

I need validate anyway if user select YES to cancel data save.