hashtag

Yes, you can use the mashshare open graph add-on which is free

Do you have any idea how? I installed it, but am still lost.
Thank you.

Please check the screenshot: https://take.ms/B3ppP

Ah, thank you. Is there a way if I want to use the same #hashtag for every post, or would I have to add it manually to each one?

Soory thats not possible at the moment. You can only define the twitter username for all posts in the mashshare settings