MacOS – ny keyboard shortcut to add bookmarks in Safari

Not built-in, but it is available through a no-restart extension called Bookmark Shortcut Keys:

Access Bookmark Toolbar items with a keyboard shortcut.

The default is Control+1-9. Change it in the preferences.

Note that you have to organize your bookmarks toolbar with up to nine of your favorite links at the beginning of the list to use it. There is no way to map a specific number to a specific link (except by the order in which they appear).

Note:
In my use, I could not get Cmd+1-9 to work because it's used to navigate to open tabs. But it did work with Alt+Shift+1-9.
I didn't use the default Control+1-9 because that's mapped to switching spaces in OS X and I didn't want to re-map it.

You can add/change any Key Command from
System Prefs > Keyboard > Shortcuts > App Shortcuts

Click the + button then add the info you require. [Later removing any custom binding will revert the app to its default binding]