Seems like we can navigate to the link dialogs first tab, the internal link tab, but not to the second tab, the external link tab. Checking mw:VisualEditor/shortcuts and mw:VisualEditor/Portal/Keyboard shortcuts it seems like this has no solution right now. It seems to me that [alt][k] or [ctrl][alt][k] would be an obvious choice. Neither of these seems to have any action in Firefox on Ubuntu.
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If you just start typing an external link ( 'http://...' ), or paste one, it will automatically switch tabs, so I'm not sure how important this is...
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Given the scarcity of available keyboard shortcuts, I'm going to decline this for now. There are already accessible ways to enter external links.