You must file Form 2553 to elect S corporation status, @carlhua.
The IRS even informs you accordingly when you apply for an EIN.
We must initially classify you as a regular corporation. In order to elect S corporation status for your organization, you will need to file Form 2553 (Election by a Small Business Corporation). Once that form has been approved, we will update your account.
See https://www.irs.gov/instructions/i2553#idm139855003794080
