You do not register for the stimulus. You either eligible or you are not eligible based on either your 2018 or 2019 filed federal tax return. Or you receive Social Security benefits or you are not required to file a tax return.
If you are not filing a 2019 tax return and you are not required to file a tax return and you do not receive Social Security benefits, then use this TurboTax website to register for the stimulus - https://turbotax.intuit.com/stimulus-check/
If you receive Social Security benefits, see this US Treasury website - https://home.treasury.gov/news/press-releases/sm967
See this IRS website for stimulus payment information - https://www.irs.gov/coronavirus/economic-impact-payment-information-center