https://www.foxnews.com/science/if-you-think-you-had-coronavirus-a-new-study-might-give-you-your-answer
Edit: Email address to participate (can not have had a positive covid-19 disease test): clinicalstudiesunit@nih.gov
They will test "up to" 10,000 people (presumabily they have some additional criteria, age, sex, where you live)