I find it easiest right-click>inspect the password field in chrome, and paste my password directly into the html value field.

Another thing to note, if you're particularly trying to load up, is you can buy I-bonds as a 'gift', separate of your individual limit. Specifically you and your spouse can buy $10k each directly, plus each buy $10k as a gift to the other (thus $40k total). You can actually buy >$10k as a gift for someone, but you can't transfer beyond $10k a year to that person, effectively increasing the minimum hold period.

Also a general note - you can enter your payment account electronically, but ever changing it requires mailing a form! So choose wisely.