A document's ft:originId is:
the document's ID + variants selected at the moment of export
The document's ID is taken from the book ID in Author-it.
For example, if the book ID is 29320, and the variants selected are, "FT_Version": "4.0", "locale": "en-US", and "audience": "public", the document's ft:originId is the following:
29320_FT_Version-4.0_locale-en_US_audience-public
It is not possible to set a custom ft:originId value.