I just got custody of a child how do I add them to my stimulus package

[xmasbaby0]

Sorry--you cannot do that at this time.  When you prepare your 2020 tax return, IF the child qualifies as your dependent for tax year 2020 (and we cannot be sure of that without more details) you will get the stimulus money with your 2020 tax return.  Right now the IRS is using information from returns filed in 2018 and 2019 for stimulus money.  If you "just" got custody in 2020, you cannot get stimulus money for that child now, nor do parents of babies born in 2020 get stimulus money for those babies in 2020.  Wait for your next tax return.

[xmasbaby0]

You say that you "just" got custody of a child---so we do not know if you will even be able to claim that child as a dependent on your 2020 return.  Who did the child live with for the time before you got physical custody?  Did the child live with you--or someone else?  The person who the child lived with for more than 183 nights in 2020 is the one who will be able to claim the child as a dependent for 2020.

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent