You can't make crosspost on it's own.
Cross post only appears, when you're several resources, avalible on CivitAI, with the same hash.
For example, if your image meatadata contains model hash, and you're reviewing, for example - style LoRA, the same window for reviewing model would apper.
This looks like that:
When you're making review post, of course.
its automatic. when you post an image that has different resources that are already in civitai then it will automatically post them as cross-post