firmware locked MacBook Air 2017

You can also take the laptop to an Apple Authorized Service Provider as well. I recommend you contact the Apple Store or AASP first to confirm exactly what paper work will be acceptable.

https://support.apple.com/en-us/HT204455

Yes, if it is forgotten and can not remember it - The Only way is to get your Original Bill of Purchase from who it was purchased from and take the computer to an Apple Store. They will required Proof of who you are, Proof of Purchase / Ownership before even considering unlocking the Firmware Password.