After you choose "Meta Key" option, you should complete the following:
1) Go to "All Posts";
2) Choose the post you would like to be displayed by our plugin;
3) Click "Edit" button;
4) Find "Related Posts" section;
5) Check "Key" radio button;
6) Update/Publish the post;
7) Repeat all these steps for each post you would like to display as related.
This article related to Relevant plugin
Additional resources: Relevant - User Guide
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