In almost all cases, the IRS will not accept an application for an ITIN unless the person has a financial connection to the United States. If the spouse is not a resident and does not receive US income, then the financial connection is created by the joint tax return. Therefore, the IRS will only accept an ITIN application if it is attached to a tax return and they will not process a separate early application.
There are a couple of exceptions under tax treaties, and you should review the instructions for the W-7 application very carefully before you mail any paperwork.
see this
https://www.irs.gov/individuals/international-taxpayers/taxpayer-identification-numbers-tin
