Depending on how you access this instance, there should be an icon for it underneath your post. At least, that is the case with the default web UI (the cross-posting icon would be looking like two partially overlapping squares). On the Jerboa app I do not see the possibility (I did not try other apps), but you could also follow the steps like your original post here (but choose the precum community). Obviously I (or other members) could do this too, but than the crosspost will not be associated directly with your account (but carry the username of the one doing that for you).