Back Button click event

Let's say that we are on AP10 and we click a button to transfer to PO10.  On PO10 there is a "Back" button to the left of Add / Change / Inquire / Delete / etc.     Clicking the Back button takes you back to AP10.     So far so good, but let's say we want to intercept the event of when the Back button is clicked and do some edits that may result in the fact that we want to keep the user on PO10, i.e., negate the Back button action.
    In the Design Studio for PO10, how can you detect when the Back button is clicked?     The FORM_OnTerminate() function will be triggered but it is too late at that point to do some edits and keep the user on PO10, it goes back to AP10 regardless.