How do you add an online certificate to LinkedIn?
Add it under "Licenses & certifications" with the issuing organization and the certificate URL; LinkedIn Learning and Coursera do it in one click. Also add the skills you learned to your Skills section; that is what recruiters actually search on.
A certificate nobody sees does nothing for your career. LinkedIn is where it becomes visible, and most platforms make it easy: LinkedIn Learning has a built-in button, Coursera and edX provide a shareable certificate URL with verification, and DataCamp and Codecademy issue shareable certificates too.
The right spot on your profile is Licenses & certifications: certificate name, issuing organization (pick the official company page so the logo shows), and the verification URL. More important still is the translation: add the concrete skills (SQL, Python, project management) to your Skills section, because recruiters filter on skills, not course titles.
The best presentation is not the certificate but the result: a post sharing what you built or learned, project included, does more than a quiet profile update. And stay selective; five relevant certificates look stronger than thirty loose badges.