To create a hyperlink to a website (without opening in a new window), be sure you are editing in “Visual” mode. (see the tabs at top right)
Highlight the snippet of text you wish to make into a hyperlink.
Click on the chain link icon on the menu. A box will pop up. Enter the URL of the website you wish to link to. Then click on the “gear wheel” icon to view link options.
The link options window will pop up. Enter the URL of the website you wish to link to. Then check the box marked “Open link in a new tab”. Click on the “Add Link” button. Be sure to click on the “Publish” or “Update” button to save the page when finished.