While Earnest does not have a principal-only payment menu option, you can schedule a payment to ensure that the funds are applied towards the principal balance of your loan. If you're enrolled in autopay, you can do this by logging in to your Earnest account and either increasing the amount of your upcoming monthly payment or scheduling an extra payment for the same day. The extra amount will go directly to principal, since the interest accrued is applied from the monthly payment.
Please note that this may work differently if you have extra interest that has accrued during a grace period, deferment, forbearance, or skipped payment. If you are not enrolled in autopay, you can choose to pay more than your minimum monthly amount, but will not be able to schedule an extra payment.