Can I add the certificate to LinkedIn or my CV?

Yes, and that is exactly what it is for. Since each certificate has its own public verification link, you can add it to your LinkedIn profile in the licenses and certifications section, or include the address on your resume so that anyone reading it can verify it.

On LinkedIn: add it under “Licenses and certifications”, with Kluppy as the issuing organization and the verification URL in the credential field.

On a resume: indicate the level achieved and your certified speed, with the verification link next to it. It is more credible than a line that says “typing: high level”.

And in a selection process: the link allows whoever is evaluating you to check it in ten seconds, without calling anyone or just taking your word for it.