You should submit your most recent tax return. If you submit your Small Business Certification Request prior to the current year’s due date for your taxes, you may submit your previous year’s tax return.
If you obtained an extension to file your taxes, then you may use your most recent return filed prior to the extension. In this scenario, you should also include your IRS Form 7004: Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns in your Small Business Certification Request.
