How to set Canonical URL

Hello
I have a web-page that is a duplicate of another on my site (with very minor variations). The duplicate page supports a prototype upgrade to an existing product.
I’m trying to work out how to ensure that search engine concentrate on the main product, while giving users an option to navigate to the prototype page.

Presume this can be done with Canonical URLs, but I can’t see how to achieve this.
Can you help?

Thanks

p.s. the prototype page was generated using Yoast Duplicate Post.
p.p.s. page currently in draft – hence no link