You'll want to send DM_SETDEFID (0x0401) to the parent dialog. The wparam will be the control id (not the handle). If you are creating your Accept/Decline/Back buttons on your custom page, the dialog is not HWNDPARENT, it is your inner dialog.
code:
!define DM_SETDEFID 0x0401
SendMessage $HWND ${DM_SETDEFID} $mui.Button.Back.Id 0