When filling in your income tax return form (Modello Redditi, Modello 730, or Modello CU for those who are exempt)

You can indicate your choice by identifying the box dedicated to the Istituto Buddista Italiano Soka Gakkai (IBISG)

3. and signing your first and last name in the box (see an example here)

To express your 8×1000 preference you only need a signature; the entity’s fiscal code is not necessary.