Resolved! Using Auth API on WHMCS - Univ Prompt?
(My edit: I somehow glanced at this thread and thought it was referencing the Univ Prompt. ¯\_(ツ)_/¯ https://community.cisco.com/t5/apis/duo-auth-api-verified-push-release-date/td-p/5035155)Obviously, it's not, so my question still stands - I will c...