If the target site is not in the list of possible migrations (what we call a site's migration path), the thing to do is flag the post for moderator attention, select 'other' and indicate where you feel the question should go.
You indicated in comments that you'd rather not add unnecessary work for moderators. That's very polite of you, however that type of flag is very easy to service. If the moderator also feels that the question would be a better fit on another site, they'll migrate it.
If you're worried that perhaps a moderator won't see your flag in time to migrate a question before the community closes it as off topic, that's also not a big concern; if the question has been closed, but was good, a moderator will re-open and migrate it. Additionally, you can leave a comment indicating that you flagged for migration to let others know.
Migration paths are established when they'll be beneficial for both sides of the path. If the site you have in mind is not an established migration path, you'll have to involve a moderator.