How to add custom canonical url tags to CMS pages?

i want to add custom canonical url tags to CMS pages.
That is, a new field “canonical” will be added to the SEO section.
If this field will not filled, then take the value from the “URL Key” field.
If this field will be filled, the “canonical” field value will be overwrited.

I found only how to add canonical tags to all cms pages, but I didn’t find how to make them custom.