How to Specify an Alternative Canonical URL in Craft CMS + SEOmatic

I have been helping a friend out with his blog which uses the Craft CMS together with the SEOmatic plugin. While I was writing an article for his blog, I noticed in his blog's HTML that the SEOmatic plugin creates a canonical link that refers to his blog instead of that of the guest. This ultimately leads to two competing canonical links which then means that both blogs are harmed in search engine rankings, especially the guest author as Google interprets the guest author's blog as plagiarizing.

This is a companion discussion topic for the original entry at