WordPress oneboxes will work using the plain (post number) permalink structure. I think the issue you’re having is the lack of OpenGraph tags on the page. An easy way to get OpenGraph tags on your site is to install the Yoast SEO plugin. Here’s an example that uses the post-number permalink structure - first with the Yoast SEO plugin enabled, and then a similar post, but with Yoast SEO disabled.
