31 CFR §363.147

You may purchase a payroll zero-percent certificate of indebtedness through your TreasuryDirect® account using your payroll savings plan. (See §§ 363.59 and 363.60 for more information on opening a payroll savings plan.) The only method of purchase for a payroll zero-percent certificate of indebtedness is a credit of funds from your employer or financial institution using the ACH method. You cannot purchase a payroll zero-percent certificate of indebtedness by using a debit from your financial institution.