This functionality is built into WordPress:
On Appearance > Menus , click on the link for Screen Options in the top right of your screen. Hidden away in the dropdown there, below the usual Show on screen checkboxes, is another line of Show advanced menu properties checkboxes.
Check the box for Link Target and then all your Menu Items will have the additional option of Link Target : “Same window or tab” or “New window or tab”. Check this box to have the link open in a new window.
Alternatively, you can install the Open External Links in New Window plugin to have all links going outside your site open in a new window automatically.
