Although you don't need a dedicated hackathon platform to run your hackathon, using one will help you to corral your participants through a journey that will help them to deliver the best possible solution to your challenge. Please take a note that all these platforms are pay-per-use.
DigiEduHack LMS free (under certain conditions) and super easy to use (available mid-August 2020)
Taikai one of the simplest to use yet most advanced platforms, with a great blockchain-based judging system. The huge plus: the "Taikai for Good" program that grants free access to the platform and all its features for educational institutions, research institutions, and non-profit organisation (with a limitation to 50 participants per hackathon)
DevPost a heavyweight player in the segment. Advanced and expensive (1500$ per month)
Agorize a hi-end, dev-oriented platform. Advanced. Price on request.
Mettl versatile and customisable platform. Prices starting at 3500$ for a 10 to 50 participants hackathon.