To allow menu items to open up in a new window, you need to change a few settings in your WordPress admin. See the steps below.
First you need enable “Link Target”. Navigate to your menu section of your admin “Appearance > Menu”. Then click the Screen Options button in the top right corner of the screen. and check the “Link Target” option
Then you can will see a new option in each menu item called “Open link in new window/tab”. Simply check that box for the menu items you want to open in a new window/tab.