Instead of tweaking in settings I use this way
Demo: Discourse Theme Creator
<script type="text/discourse-plugin" version="0.8">
api.onPageChange((url, title) => {
// Automatically open external links in new tab or window
var pc = 1;
if ($("html").hasClass("mobile-device")) pc = 0;
var links = document.links;
for (var i = 0, linksLength = links.length; i < linksLength; i++) {
if (links[i].hostname != window.location.hostname) {
if (pc) links[i].target = '_blank'; else links[i].target = '_self';
}
}
});
</script>