The authors of the the VCL helpfully added multi monitor “support” a few versions back. Somewhat less helpfully, this support is very limited – it has no way to say “create form X on monitor 3”, which is a very useful thing to do in some kinds of applications – those intended to run on a multimon system in a specific configuration.
You can readily move forms around (with .Left, .Top, etc.) on to specific monitors after a Form is open, but not while it is opening (in OnShow or OnCreate), because a fine method TCustomForm.SetWindowToMonitor will jump in and undo your work.
Not surprisingly, most of the obvious methods to override to fix this, are not virtual. (An aside – I think the bias toward final methods in Delphi, C#, and some other languages, is a poor idea, and I think the guideline of “design for overriding, or prevent it” is even worse.) With some looking, I found a workaround, shown here somewhat mangled by WordPress:
procedure SetBounds(ALeft, ATop, AWidth, AHeight: Integer); override;
procedure TSomeForm.SetBounds(ALeft, ATop, AWidth, AHeight: Integer);
begin if FSetBoundsEnabled then inherited;
FSetBoundsEnabled will be False by default. Sometime after your form is loaded (use a Timer or a windows message to signal this), set FSetBoundsEnabled := True then set your .Left and .Top to put your form whereever it needs to be.