The solution has been posted in the Community forums: Open external links in a new window. Take note that according to most usability experts, opening links in a new browser tab is considered bad practice.

We invite you to read the article Seven tricks that Web users don't know, particularly the section Second browser windows: “Avoid spawning multiple browser windows if at all possible — taking the "Back" button away from users can make their experience so painful that it usually far outweighs whatever benefit you're trying to provide. One common theory in favor of spawning the second window is that it keeps users from leaving your site, but ironically it may have just the opposite effect by preventing them from returning when they want to.”