I live overseas. I am a US citizen. I have birth to my youngest son in the Marshall Islands. He doesnt have a US ssn. How can I claim him?

To get a Social Security Number overseas you need to make an appointment with the US Embassy or Consulate to provide an application for an SSN.

There are a number of significant tax benefits for dependent children that you only qualify for if you file an on-time return with SSNs for your children.  

If you get the SSNs later and file an amended return, these benefits won't be paid.  For this year, an on-time return means filed before July 15, 2020; OR filed before October 15 but only if you have a valid extension.  

To get a valid extension until October 15, you must file form 4868.  Usually, this form can't be e-filed after the regular deadline of April 15.  I don't know if this has been extended.  Overseas taxpayers get an automatic 2 month extension to June 15, and you can file form 4868 by mail after April 15 and before June 15 by using the overseas extension method (which I think, is to write your request on the top of the form, but check the instructions.)  

This year, it might be possible to still e-file a valid form 4686 even though it is after April 15 because the mail filing deadline was extended.  Either way, if you plan to file a tax return with your dependents, you need a valid extension to October 15 and to actually get the SSNs before you file an on time return.  https://www.irs.gov/forms-pubs/extension-of-time-to-file-your-tax-return.

This question was previously answered by Opus 17