Manifest Yourself!
To add a link, first you have to select existing text (a fragment you want to be the link) and click the "Link" button - in the top menu:
Now a window appears, in which you have to type in the URL address of the website you want to link:
To delete the link, click the selected text, once again open the URL address window and delete the present address
See also: How to add a graphical link