How can I add a menu item that links to an outside website?

Hello, hope everyone had a good weekend!
I'm using Marketpress with the Gridmarket child theme. As suggested, I have 2 separate Wordpress installations for the Store and for the main website associated with the store. I now need have a (primary) menu item on the Store page that links back to the main website. I feel dumb but... it seems the only way to do that is to go to the HTML editor of the page and add a new

  • item right in the source code. Is that right or am I missing something in the Menu theme? Thanks!