Basically your post is already "posted" there. Anyone with a mastodon account on an instance that federates can see it. They could either paste on their mastodon search interface the url to this post and see it appear as a mastodon message. They could then reply on mastodon and it would show for you here on your lemmy instance.

They could also follow your account on mastodon or a community just as if you were a mastodon user and then see your posts on their timeline.

If some user follows you or community it will get federated into mastodon. Otherwise you can't relly push the post into mastodon.

If you mention a Lemmy community in a post on Mastodon, it appears as a post in that community. Like this post on Mastodon: it also shows up in the Lemmy community "Linux", and many if the comments are from Lemmy users: https://social.edu.nl/@foxy/111937325118529631