You can submit your application through the portal here: https://www.herox.com/SolarDesalination/phase/1016/entry/create
I would refer you to the official rules for more information on this topic:https://americanmadechallenges.org/solardesalination/docs/Solar_Desal_Prize_Official_Rules.pdf
The application consists of two parts. The written portion and the video summary. Per the official rules, the written submission may be viewed by the DOE, Prize Administrator, and judges and reviewers for purposes of the contests, including but not limited to screening and evaluation purposes.
The video portion of the application summarizes your proposal in 90 seconds or less. Teams are discouraged from including any sensitive information in their videos, as they will be made publicly available.