According to various online reports, melatonin can help reduce risk of COVID-19 with some claims to be able to use melatonin to treat COVID-19.
See this article in ScienceDaily
In terms of the use of mouthwash to kill COVID-19, 30 seconds,
Oral rinses containing 0.07% of the ingredient cetylpyridinium chloride show “promising signs” of reducing COVID-19, reducing COVID-19 in saliva, research shows.
With the surge in cases in the US, and a run on toilet paper, now we will have a run on melatonin and mouthwash.