IF YOU HAVE TROUBLE WITH THIS, YOU DIDN'T FOLLOW INSTRUCTIONS. YOU SUCK. TRY AGAIN, THANKS.
I found this solution while looking at the OptiFine github forum and made the video for a couple people I play with. I thought others might like to know as well!
For those that can't be bothered to hear my voice and listen to words:
click after bracket
press space
press enter
press tab
paste (including quotation marks) "net.minecraft.client.main.Main"
immediately add a comma after ending quotation mark
save
that's it
spacing at the end of the video didn't matter, loaded and discord recognized it all the same.
It's not rocket science. Work from 1.15 to 1.18 at minimum. Comment from London Breakfast below works for 1.12 and maybe lower? Give it try and let everyone know