The IRS will most likely send you your stimulus check the same way they send you your SSI, but you can also click the link below, scroll down on the screen where it says "If you don't need to file" and register there.
Do I have to fill out a 1099 or something to get stimulas check in my direct Express account from social security
No you do not have to file anything since you receive Social Security benefits.
If you receive Social Security benefits, see this US Treasury website - https://home.treasury.gov/news/press-releases/sm967
No, the check will be paid the same way you get the SSI.