Hi, wanna do experiment across the fediverse? Here is a community dedicated to that: !testfediverse
From my experience with Miskey forks, which behave almost like mastodon, to post from the twittoverse to Lemmy, you need to make sure the community is federated to your instance. To do that, open it like using the same url pattern for a remote user. Example with Sharkey : go to sharkey.world/@firefox.
Then write a note with a mention to the community just like it was a user. Do not mention more than one community because it will post only on one.
The first paragraph will be repeated as you title so avoid mentions and hashtags there.
On !testfediverse we have tutorial on how to do that and also how to post from Lemmy to Mastodon. It is in french but you could use a translator or ask mods for advice.