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Links are added to rell = "nofollow" and how to add and target = "_ blank" ?
You should use a plugin that does just that: https://open.vanillaforums.com/addon/target_blank-plugin
Target Blank plugin not working with Vanilla 2.6 version
Now you have a new addon for this, it's called Link Out: https://open.vanillaforums.com/addon/linkout-plugin