Hi Vitali, thanks for the request! Good news: this is now available natively since Strapi v5.51.2, so no plugin needed anymore.

In the Content-Type Builder, you can now set a relation field as required in its advanced settings, just like other fields.

Release note: https://github.com/strapi/strapi/releases/tag/v5.51.2

I'm marking this one as Complete. Let us know how it works for you!