Вопрос: в какой среде? В CBuilder:
helpTCustomForm::DefaultMonitor
Specifies the monitor on which the form appears.

enum TDefaultMonitor { dmDesktop, dmPrimary, dmMainForm, dmActiveForm };
__property TDefaultMonitor DefaultMonitor = {read=FDefaultMonitor, write=FDefaultMonitor, stored=IsForm, default=3};

Description

Use DefaultMonitor to associate a form with a particular monitor in a multi-monitor application. The following table lists the possible values:

Value Meaning

dmDesktop No attempt is made to position the form on a specific monitor.
dmPrimary The form is positioned on the first monitor listed in the global screen object’s Monitors property.
dmMainForm The form appears on the same monitor as the application’s main form.
dmActiveForm The form appears on the same monitor as the currently active form.

Note: DefaultMonitor has no effect if the application does not have a main form.