You can't.
If the child is eligible for an SSN, apply for it now. Wait for it to come before filing a tax return. You should also submit an automatic extension request to extend your filing deadline to October 15. There are significant tax benefits for a dependent that are only paid if you file an on-time tax return with an SSN. (For 2020, on-time means filed before July 15, or filed before October 15 if you got the extension before July 15.)
If the child is not eligible for an SSN, you need to apply for an ITIN, international tax ID number. This is done with form W-7. Start by using a fake ITIN to prepare your return, try using 999-88-9999. You can't e-file. Print the return and use white-out to cover the fake SSN. Prepare the W-7 application for the dependent and attach any proof required by the instructions. Mail the W-7, the proof, and the tax return, to the IRS address for the ITIN application. Use a mail service with tracking and proof of delivery. After the IRS assigns an ITIN, they will process the tax return.
