Hi Daniel,
The feature you’re looking for, where users are allowed to post but cannot see the category itself, is not directly supported by the default Discourse settings.
You could enable the Accept emails from anonymous users with no accounts for your category to allow users without a Discourse account to post to it via email.
However, if the user does have an account, your current security settings will prevent them from creating posts, unless the Create checkbox under security is checked for a group that they belong to - keeping in mind that they would need to be able to See the category in order to create topics in the category.