How to set nomodeset GRUB2 before I've installed Ubuntu

You put nomodeset between the quiet and splash part.

You need to enter the quiet splash text using the arrow keys, as i circled in this picture:

And you have to type in between quiet and splash (or you may say, splash and quiet.) the text nomodeset, so now it looks like this:

getparams 'Try Ubuntu without installing'

       set gfxpoyload=keep
       linux /casper/vmlinux.efi file=/cdrom/preseed/ubuntu.seed boot=casper $ quiet nomodeset splash -
       initrd /casper/initrd.lz

(I'm not sure if the picture says "set gfxpoyload=keep" beacuse it's confusing to see the text.)