You can set main menu items to open up in a new window using the default WordPress option ‘Link Target‘. See the the steps below.
1. To enable the option, please login to your WordPress Dashboard and navigate to Appearance > Menu. In that new section, click the Screen Options button in the top right corner of the screen and select ‘Link Target‘.
2. You will see a new option in each menu item called ‘Open link in new window/tab’, check that box for the menu items you want to open in a new window/tab.
3. Once done, click ‘Save Menu‘.