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Focus instead on becoming a valuable member of these communities and building rapport with other members.
Then, link to your ebook from your Facebook or Reddit profile so people who check out your page can find their way back to it, and subtly mention that you’ve written an ebook in your comments when you get the opportunity to do so—but never make it obvious that you’re trying to promote your book.
Other communities may be more open to self-promotion. For example, groups for new authors to get feedback. These are great places to find beta readers for your ebook in particular.