Click "Add Content" -->"Default" -->"External Link"
This brings up the external link modal. From there you go to “Link Name”: Name the external link whatever you would like the link in your menu to say.
"Placement Folder": is where you want the folder located. Make sure you have the correct folder selected.
"Link Settings": Paste the address of the external site (ex: https://www.google.com/).
Click "Save & Preview" then Click "Submit"
It'll be a tab on the side navigation bar.