As people who study native american languages know, Linux/ubuntu has little support for native keyboard layouts, which appear to all be made for windows and mac. For those studying Choctaw or a language that has the underline bar diacritic marker, a workaround for the underline I found is this for linux:
Install xdotool. Then go into the keyboard settings for your computer and add this line as a shortcut key (I attached it to the minus key on my number pad). Afterwards, if you enter any letter and then hit that minus key it will underline the letter Choctaw style.
sh -c “sleep 0.2 && xdotool key ctrl+shift+u type 331 && xdotool key Return”