Assuming that you qualify for the extension for the working capital safe harbor from 31 to 55 months, you do not need to file any extension or otherwise provide any evidence to the IRS; it is automatic. You qualify for this extension if (i) your working capital assets were intended to be covered by the safe harbor before June 30, 2021; or (ii) your QOZB is located in a qualified opportunity zone within a federally declared disaster.