dynamic cross-domain rel=canonical in YOAST free version

We have YOAST free version on WP4.1 at let’s say https://www.myhelpsite.com. I was asked to do rel=canonical to a separate domain as follows: we have an article published in WP on https://www.myhelpsite.com/article-slug/. They want my header to contain <link rel=”canonical” ref=”https://www.mymainsite.com/help/article-slug/&#8221; />.
In the whole damn web I couldn’t find one article clearly stating WHAT to change and WHERE, so: what code do I write and where in order to have my header entry rendered corectly?

I should have also mentioned we have WPML installed and it’s got “Add links to the original content with rel=”canonical” attributes.” checked under SEO Options.

Thanks a lot!