Open the original post, then hit this cross-post button:
Then you can select the community to cross-post to. Your new post will then retain the proper cross-posting metadata so others can find the OP more easily. The power of the Fediverse can also be leveraged to cross-post across instances too, which I did with this cross-post of mine.
Aha! I should have looked more closely at the various icons, did seem like something that ought to be there and it is ...I'll delete this and try that. Thanks!