Ответ, по-видимому, содержится в комментарии в файле FIRDLDefaultRetrievalProcessV2.m для iOS SDK:
// Reason for this string to ensure that only FDL links, copied to clipboard by AppPreview Page
// JavaScript code, are recognized and used in copy-unique-match process. If user copied FDL to
// clipboard by himself, that link must not be used in copy-unique-match process.
// This constant must be kept in sync with constant in the server version at
// durabledeeplink/click/ios/click_page.js
Затем SDK считывает буфер обмена с помощью UIPasteboard.