Oh, yes. The only way is either type the part of the tag and see what existing tags AO3 offers as autocomplete, or just remember popular tags from other works. For "Shanshu", for instance, you could add "Post-series", "Post-chosen" and "amnesia" tags. And probably add "Gen" as a genre (e.g. "F/M" and "Gen" instead of solely "F/M").
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For "Shanshu", for instance, you could add "Post-series", "Post-chosen" and "amnesia" tags. And probably add "Gen" as a genre (e.g. "F/M" and "Gen" instead of solely "F/M").