You actually can’t. I think you should be able to, but currently you can’t. You can post from Masto to Lemmy by tagging the Lemmy community in the top level post, but tagging in a reply does nothing

In fact actually you can’t even really crosspost from Lemmy to Lemmy. Posts that link to the same thing are lists as cross posts, but aren’t actually directly related. Text-only posts can’t be recognized as crossposts at all