Not all producers offer terms. You may want to discuss with them directly before requesting terms through FarmersWeb.
To request terms from a producer through your FarmersWeb account, visit the producer's profile page. Under their farm photo, you'll see 4 tabs. Click on Terms. Click on the green submit request button to request terms from this farm. You can also request terms from farms that accept terms on the checkout page. Simply click the orange button below Offline Terms to the right of the order total.