Comment 16 for bug 475864

I just managed to get kickstart working with an expert recipe in 18.04. First, kickstart doesn't support the backslash syntax, so the entire recipe needs to be on one line. Second, as mentioned in #9, you need to surround the whole recipe with single quotes. Your preseed line should look like this:

preseed partman-auto/expert_recipe 'string boot-root :: 500 500 500 ext4 $primary{ } $bootable{ } method{ format } format{ } use_filesystem{ } filesystem{ ext4 } mountpoint{ /boot } .'