You can add this code snippet to your theme that will cause any external links on your online store to open in a new browser tab. This is a variation of the Shopify customization guide that does not require jQuery. https://help.shopify.com/en/themes/customization/navigation/open-external-links-in-new-browser

external-links-in-new-tab.js

var links = document.links;
for (var i = 0, linksLength = links.length; i < linksLength; i++) {
  if (links[i].hostname != window.location.hostname) {
    links[i].target = '_blank';
  } 
}