Select the default window to be opened at startup

I'm working on a MacOS X (Objective-C / Cocoa) application which basically works this way : first a window is opened, requesting user's username / password. If credentials are valid, the main application is displayed.

This is what I do to open my credential window :

@implementation BetaseriesDesktopAppDelegate
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- (void)applicationDidFinishLaunching:(NSNotification *)aNotification 
{
    authWindow = [[AuthenticateWindow alloc] init];
    [authWindow makeKeyAndOrderFront:nil];
    [authWindow becomeFirstResponder];
}

- (void)login:(id)sender
{
    NSLog(@"Login");
    User *user = [[User alloc] init];

}

@end

Problem is, the main window if opened too at application startup. How can it prevent it to do so? Is it a property I must set in Interface Builder or do I have to do that in my applicationDidFinishLaunching method? Something else?

In Interface Builder, select the window. In the Window Attributes info pane, deselect the option "Visible At Launch".