The purpose of a <link rel=canonical> is to show search engines which duplicate or similar content is the original. Currently you cannot specify this value in Composer, it is automatically generated. There are times when one site duplicates/shares the content from another, and showing that other site's canonical URL is necessary for good SEO.
Proposal
Add a new field to the Meta tab in composer called "SEO Canonical URL". The editor can enter any URL in this field, and it only errors if the URL is invalid format. The URL can be either relative or absolute, internal or external.
If this field is empty, the frontend can populate the <link rel=canonical> as usual.
If this field is not empty, the frontend can populate the <link rel=canonical> with the value entered in the field.
A thought: instead of altering the canonical_url ANS, add a new field called seo_canonical_url
Guest
We would like to see this built into Composer as well. We're going to use an unused field for this, but it's not an ideal experience.
Adding more support for this idea. We have a time-sensitive feed of content that we want to use, but are unable to do so because of the lack of support for defining the originating URL as the canonical URL of the content.
We need to ensure that the SEO value of the content isn't inadvertently hurt by identical versions of the content being published on various websites.
Thanks to Bridgette for the proposal.
Just noting here that Bonnier sites would benefit from this greatly. We syndicated from other sites often via our legacy CMS and are hesitant to do so without an SEO canonical URL field to attribute proper credit to the original article.