Custom Menus is really is a cool feature in WordPress which is introduced in version 3.0. Through Custom Menu, you can create a Navigation, as per your wish. You can rearrange the pages, posts, categories or even use any external link and create a navigation or use it in sidebar. Before version 3.0, there was no such feature.
With Menus, you can create a navigation, or just add that menu to the sidebar. Now when adding external URL, the effect in frontend would be that that particular URL will open in the same window. Some may don’t want that particular link to open in the same window and may even want it to open in new window. No, you don’t need to add target=_blank anywhere for this purpose. The solution is really simple.
How to enable custom menu to open in new Window
Go to Menus, and from the top right side, click on Screen Options.
As shown in the screenshot above, click on Link Target under the Show advanced menu properties.
Now when adding a new custom link, category or page, you will be able to see a new option named as Link Target. Select New Window or Tab from drop down menu. (Refer below screenshot)
So now whatever you add, custom menu will open link in new window. Hope you find this small tutorial as helpful.