The same way you claim a dependent who is under 18. In My Info you enter the information about the dependent including the Social Security number. For a dependent over the age of 16, you will get the credit for other dependents. You will not get stimulus money. And the person who you claimed will not get stimulus money, since dependents are not eligible to get a stimulus check.
CREDIT FOR OTHER DEPENDENTS
Add them under My Info and answer the questions. Either has to be a full time student under 24 or make less than 4,200 income for the year among other qualifications.
Who can I claim as a dependent?