Afaik, you cannot fax the form. You can only mail by paper or e-file.I mean PDF digital signature then fax it to the IRS.
They use a software and do e-fileI paid an accountant to do mine and he did a digitally signed signature. Apparently it's fine.
Where To File
File Form 1120-F with the Internal Revenue Service Center, P.O. Box 409101, Ogden, UT 84409.
Corporations can generally electronically file (e-file)
Waiver requests can also be faxed to 877-477-0575. Contact the e-Help Desk at 866-255-0654 for questions regarding the waiver procedures or process.