Zotero extension BetterBibTex use the following citekey format on its configuration
shorttitle(3,3) + auth + year
the format of citekey is separated by lower camelCase
titleAuthorYear
How can I change this configuration lowerCamelCase to use undescores as separators?
title_author_year
auth.lower + '_' + shorttitle(3,0) + '_' + year
might do it.shorttitle(3,3).lower.condense(_) + '_' + auth.lower.condense(_) + '_' + year