How to crosspost? - Lemmy.world

I see people cross-posting but I don’t know how they are doing it. All I see for posting options on the lemmy interface is a single community I can select.

In the web UI, there’s a “crosspost” icon under the post header, right between the one for “save” and the one for “view source”.
Does anyone read the rules for this community?
Yes, though I didn’t conceive of it as a support question - I will post there, thanks.

It breaks rules 1 and 3. And this kind of question is literally in the example they provide:

“How do I block a user?”

I understand, hence my prior reply. Thank you for making it additionally clear.