Can't add "Read More" link in Blog with Summary View

Can't add "Read More" link in Blog with Summary View

I have "summary view" enabled in Blog Manager preferences, however I cannot see any option to add a "read more" link after typing 750 characters in a blog post.  Behaviour in blog posting is the same whether or not "summary view" is checked.  Where and how does the option to select a "read more" link?