“mainEntity” is missing on FAQ page

Hi! A couple days ago I created a “FAQ” page on my website. However, today I got an error from Google Search Console about “mainEntity” (that it’s missing).

I read a lot of support topics in Yoast Seo section, and understood, that I have the same problem, as here – https://www.ads-software.com/support/topic/mainentity-missing-on-faq-page/ . My “FAQ” is located in a “Page” type. So as I see in the topic, pages do not get a MainEntityofPage schema.

But I unloaded Gutenberg a long time ago – I use a Classic editor instead. So I can’t use Yoast FAQBlock. It’s sad, that you don’t have it for Classic editor.

So I understood, that the only way to add “mainEntity” to FAQ, is to force Yoast to output the mainEntityofPage. However, I read a lot of your documentation, but I can’t understand, what code exactly do I need to add (but I know, where).

I am a pretty experienced user, so I know how to return everything back, if someone goes wrong. So I hope for your help with code, or at least to show me, where is an example of adding “mainEntity” to a certaing page.

Thanks in advance!

The page I need help with: [log in to see the link]