First, highlight the text you wish to link, then click the link icon in the post editor toolbar. A little box will pop up for you to enter your link.
If you’re linking to a blog post within your own site, you can start typing the name of the post, and the box will search for the post. When you select the post, it will automatically add the link for you.
If you want to link to something that’s not on your own site, type in the URL in the field, then hit the gear icon to open up additional options. In the popup window, you will see a checkbox to open the link in a new tab.
See it in action below!