It may be that you want to have a menu item that takes the visitor to a web page that is not in your Joomla installation. You can create menu items that go to pages outside your Joomla setup.

Menu Item Menu for External URL

This web page might be an external URL to a website other than your own or to an internal URL that still resides in your own domain. 

Go to Menu Manager and select the menu where you want this item to appear.

Click on New to create a new menu item.

It the Menu Title text area, type in what you want to appear in the menu for this link.

A popup dialog box appears with all the possible Menu Item types.

Click on System Links, then select External URL (this is also used for separated pages in your own domain).

In the text box next to link, type the URL for this page. This would be something like:

There are not a lot of tabs or options for this Menu Item type, but review the settings in the different tabs and be sure that they are what you want.

Click on Save and test that this did indeed show up in the menu and that it works properly.

Keep in mind that when the user clicks on this menu item, it will not have the Joomla menu to return to your main site so you may want to provide a link to go back to your site if you have control of the content of this web page.

You may need to clear the cache and/or disable the System - cache plugin if this link doesn't appear in the menu.