The headword, attested only here, is the aorist passive participle, neuter nominative/accusative singular, of the doubly compound verb propareggua/w, I vouchsafe in advance. For the root verb pareggua/w, used especially of passwords and other military commands, see generally LSJ s.v.
[1] The gloss (in the same form as the headword) comes from the uncommon compound verb proparagge/llw; cf. Sophocles s.v., citing Cassius Dio 46.41.4, where its aorist active participle, masculine nominative singular (proparaggei/las) occurs.
[2] Quotation (transmitted, in Adler's view, via the Excerpta Constantini Porphyrogeniti) unidentifiable.