Hello

Privacy selector element in UNA forms is a custom element which doesn't allow to change default value via Forms builder. On the other hand privacy selector uses privacy object which determines the selector's behaviour. Unfortunately, there is no any interface in Studio to manage such objects. So, if you are familiar with MySQL you may change the default value via UNA database -> sys_objects_privacy table -> default_group field. 

In case of Articles app, you need to find a row with 'aqb_articles_allow_comment_to' name and replace 2 with 3 in default_group field. Then you need to clear UNA cache in Studio -> Dashboard -> Cache and test.

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Best Regards
Anton