If you want a menu tab to link directly to another website/webpage, then follow these instructions:
1) Navigate to Appearance and select Menus
2) Navigate to Custom Links on the left and type in the URL you want to have for this menu item, add Link Text to specify what the tab will say, and then click Add to Menu
3) If you would like to control whether your menu link opens in a new tab when clicked, check the Open link in new tab option within the menu item
4) If you do not see this option, then click on Screen Options near the top and check the Link Target advanced property
Note: all external links should open a new window.