You basically need the API credentials. To get these go to your twitter's developer settings and setup an app from there. Add them to "/admin/addons/twitter" for your instance. On the same page you'll find more info about setting it up.

Thanks @tio and @Steffen K9 🍮! It seems that my connection was successful. At least I can see my account on the social media -> #Twitter settings now. And I receive the messages of the people I am following. However, I sent a 'public' post with 'post to Twitter' selected and still cannot see the post on Twitter ~10 Minutes later. Is there anything missing?

You are completely right @Steffen K9 🍮! My Twitter Dev dashboard states "Created with Read Only permissions" and the doc tells me that I can configure Read & Write in my App. I tested the development and production level of my App. Is there any additional setting in Friendica (I chose posting/default posting to Twitter)?

You are completely right @Steffen K9 🍮! My Twitter Dev dashboard states "Created with Read Only permissions" and the doc tells me that I can configure Read & Write in my App. I tested the development and production level of my App. Is there any additional setting in Friendica (I chose posting/default posting to Twitter)?

I found it!!! I needed to configure in the Twitter Developers platform the OAuth 1.0a access with reading, writing, and Direct message permission; I recreated the secrets, reconnected, and now it works. Hopefully, this little addon helps me move some of my followers to the #Fediverse . Thanks a lot to all !Friendica Support@Steffen K9 🍮 and @tio