This question seems to have the answer to your question: https://answers.atlassian.com/questions/74297.
Particularly, one answer gives this link: https://confluence.atlassian.com/display/CONFKB/How+to+Force+Links+to+Open+in+a+New+Window, which worked for me. However, where it says to go to 'Administration >> Custom HTML', it is actually 'cog icon >> General Configuration >> Custom HTML', under 'Look and Feel'. It will apply to every page in every space, but you can probably apply to a single space if you can find 'Custom HTML' in its configuration page.
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