WordPress.org

How do I override a post or page canonical with a custom URL?

You can do so by adding new custom field named "canonical" and set it to the URL desired.

I can't get it to work! I'm frustrated!

Please take a look at documentation available on the plugin page to see if any of them can help you. If not, feel free to post your issues on the appropriate plugin support forum. I will try my best to help you resolve any issues that you are having.