I believe any two posts linking to the same URL are treated as a cross-post, and that is discovered at instance-level. There's nothing more to it than creating two posts in different communities with the same link.

ETA: But to answer your question directly, there's no support for doing that as a single action with Jerboa yet.

src: https://github.com/LemmyNet/lemmy/blob/63d3759c481ff2d7594d391ae86e881e2aeca56d/crates/api_common/src/post.rs#L61

The setting is tied to your instance account, because the backend delivers the results with the specified sorting. If you change it via the browser, it should be reflected in Jerboa.

Protip: Unselecting all has the same effect.