Lemmy dev here. Making the same post to multiple communities is not possible, so you need to post multiple times tagging a different community each time. Links are taken from Activitypub attachment, but Mastodon only seems to support image attachments. So it is not possible to add other types of links unless Mastodon adds an option for that. Markdown would also have to be supported by Mastodon.
There are various other microblogging platforms on the Fediverse which may support these features and may be better suited to your purpose. For example Hometown, Pleroma, Misskey or Mitra to name just a few.
A
