It needs to be a regular template in the system just like the template you would use on the front end. Inside the Channel settings there is a “Preview URL” option you need toi fill out with the template path:
Preview URL Template path or route to use for previewing. Use {url_title} or {entry_id} for single-entry templates. i.e. blog/entry/{url_title}
Once you have that set correctly then your preview should work.
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