Based on the competition wording, verbal permission may be enough. Their rule says you must be able to confirm the person gave permission; it does not specifically require a signed model release.

In general, a street portrait made with the subject’s knowledge and consent is less likely to raise privacy issues than a photo taken secretly, especially in a public place. That said, contests may still be cautious, particularly if winning images will be published, exhibited, or used in promotion.

So: yes, it is likely valid to enter if the subject clearly agreed to be photographed, but a written release is the safest option whenever possible. If the competition may publish or heavily promote the image, getting written permission would reduce risk. If you can still contact the subject, asking for a simple written release is wise.