To make a charitable donation through Google Assistant, you must enable Google Assistant Payments. To add a credit card or debit card, open up Google Assistant, tap the compass icon and clock on the three dot overflow menu in the upper right corner. Open the settings and in the Personal Info tab, tap on Payments. While Google Home smart speakers and the Google Home Hub can start the process, both require that you confirm the donation on your handset.
Remember, your donation could be tax deductible for those in the U.S. Consult your accountant.