Gotta apply for one - login into your IRS account and apply for one there.

https://www.irs.gov/e-file-providers/become-an-authorized-e-file-provider

Then you will not be e-filing any returns. If you haven't applied to the IRS for the EFIN, it's a long process which, given everything else going on in IRS land, you may not be approved in time for this filing season.