add link from menu button to open new browser window

I am trying to create a link to a page on my other website from a button on my menu/nav bar named Gallery Art. It should link to this page and not go to a blog page or any new WordPress page. How do I do this?

I’m on the Menus page now. I kind of have an idea but this program seems so touchy that if I do something wrong I have the feeling the whole website could blow up. Thanks.