To create a link to a resource outside of a Drupal site, edit a node and do the following:

Add your text into the text editor.

Select the text you want to use for your link.

Click the Linkit toolbar button.

The Linkit dialog box will open.

Link URL field = add the full URL of the resource you want to link to. You must include the protocol. (Without the protocol ("http://), the system assumes that you're trying to create a relative URL.)

Click the Insert Link button.

Your link will now appear. Please note that the link will not be active until you save the node.