Step 1: Open the article in edit mode
Step 2: Click on the down arrow icon on the editor header for more options. Select "Draft Settings" for new draft or "Article Settings" for already published article.
Step 3: From the popup, scroll down and locate the "Are you republishing?" section
Step 4: Click the "Add Original URL" button and enter the URL you want to use as the canonical link.
The canonical link will be saved automatically.