Yes, your voucher or Mobility Budget is automatically used to pay for FREE NOW trips in the REACH NOW app. Please make sure that
a) you are logged into the REACH NOW app with the account that the voucher/Mobility Budget was added to. You can check this in the menu under "Add promo code".
b) you select a payment method that can be used with your voucher/Mobility Budget. You adjust this setting yourself in the menu under "Payment data" for your individual payment methods.
c) you do not use the own app of FREE NOW for booking.