When all else fails, read the instructions. In this case the instructions for 8858

Electronic Filing of Form 8858
If you file your income tax return
electronically, see the instructions for
your income tax return for general
information about electronic filing.
Note. If you are filing Form 1120 or
1065 electronically, you must attach
Form 8858 electronically (as an
attachment to your electronically filed
Form 1120, Form 1065, Form 5471, or
Form 8865). If you are filing Form 1040,
Form 1040-SR, or Form 1041
electronically (with or without a Form
5471 or Form 8865), attach Form 8858
to the applicable Form 8453