The first dated example of the humanistic script invented by the Italian scholar, writer and humanist, Poggio Bracciollini, is the copy of Cicero's Epistles to Atticus written in formal "antiqua" and preserved in the Staatsbibliothek zu Berlin (Berol. Hamilton lat. 166). This carries the subscription "Scriptsit Poggius anno domini MCCCCVIII a mundi vero creatione VI mil. et DCVII."
Regarding the origin of Poggio's humanistic script, Ullman (p. 59) mentioned the manuscript of De verecundia that appears to have been written by Poggio between 1402-03 for its author, the humanist and man of letters Coluccio Salutati. Thus, this manuscript is "the very first datable example of humanistic script" (Morison, Politics and Script. . . . Barker ed.  267). Unlike the Cicero, however, the date of De Verecundia is based on scholarly argument rather than concrete evidence.
Ullman, The Origin and Development of Humanistic Script (1974) 27, 59.
In September 2020 I was unable to find illustrations of the specific manuscripts cited above. Instead I reproduced spectacular examples of manuscripts in the Laurentian Library that include Poggio's signature. Those are from Roberta Ricci, "Umanesimo letterario, riforma grafica: Poggio Baraccioline editore, filogo e copista," NEMLA Italian Studies XXXVIII (2016) 245ff.