Help an italian APE

hello everyone, I need help I have xxx shares to transfer from my bank in Italy, to my computershare account, I can do everything but the "medallion guarantee stamp" what is it and how can I find it in Italy? HELP PLEASE, the timing for letters by post from Italy is really long. I need your help!

PS: se c'è qualcuno di italiano che ha la pazienza di scrivermi in privato mi aiuterebbe molto