1) Put this in your Ghost site's Code Injection Site Footer:

<script type="text/javascript">
var links = document.querySelectorAll('a');
links.forEach((link) => {
    var a = new RegExp('/' + window.location.host + '/');
    if(!a.test(link.href)) {
      	link.addEventListener('click', (event) => {
            event.preventDefault();
            event.stopPropagation();
            window.open(link.href, '_blank');
        });
    }
});
</script>

(From Aileen Nowak at https://forum.ghost.org/t/i-want-my-external-links-to-open-in-a-new-tab/9830/9)

or...

2) Put this in your Ghost site's Code Injection Site Footer:

<script>
[...document.querySelectorAll("a[href*='?xgtab&']")].forEach(link => {
link.setAttribute("target", "moredetail");
});
</script>

(From David Darnes at https://forum.ghost.org/t/i-want-my-external-links-to-open-in-a-new-tab/9830/9)

or...

3) Put this in your Ghost site's Code Injection Site Footer:

<script>
  $( document ).ready(function() {
  	$(".post-content a").attr("target","moredetail");
  });
</script>

(Note: you might have to change the above .post-content to match the CSS selector for your specific theme. For example, on one of my sites I needed to change the .post-content to .content)

or...

4) Make those external links open in an Ajax Slide-In Panel.

or...

5) Paste a code snippet from the article below into your Ghost site's Code Injection Site Footer, or at the bottom of default.hbs just above {{{block "scripts"}}}. (Either way, do put the code in between <script> and </script> tags or it won't work.)

Use JavaScript to Make External Links Open in a New Window
This is a quick post showing how to use JavaScript to make links to external websites open in a new window (or tab) instead of in the current window. This is...

Here's how it should look if you paste the jQuery from the above article into your site's footer code injection...