Ah, now I understand.
https://example.com/preview?documentId=:document_id&token=:https_urls
Use the full url you get in the token param as token in this query:
https://platform.cdn.prismic.io/api/v2/documents/search?ref=:token
and see all the documents, limit to the document you got with :document_id and you have the document you need.
This is sooo simple, but the explanation you gave was way more complex 
.