How do I tell Logic to open new windows always on my external monitor (when available)?

There is a thing in OSX that is called "key focus" or "focus".

When you type on your keyboard (text or any keyboard shortcut), the OS has to know where to send that command to if you have multiple windows open (or multiple applications). The answer is, it directs the command to the window that has "key focus".

The key focus window is the one with the active window title bar (not grayed out) and the colored three buttons in the upper left corner (unfortunately, Logic doesn't use colors).

Now to Logic:

Logic follows the same rules. However, you have to pay attention to multi-pane windows like the Arrange window. This is a window with more windows (panes) inside. Now the question is, which window pane inside that window has key focus. If you look closely, then you see that the window frame changes their shade slightly (sometimes very hard to see). Pay especially attention to plugins. They also have key focus when selected and can receive keystrokes.

Finally to your question.

You cannot set the preferences on which monitor screen Logic opens a window. This is determined by the location of the window that has currently key focus. Any new window and also alert windows and dialog windows (Save, Open) will open on that screen.

So if you want a window to open on you external screen, click on a Logic window on that screen.

I have five monitors on my Logic setup and I use a QuicKeys macro (push one button) that sets the key focus to my main monitor and now any new window will open on that screen screen.