How to write a keyboard symbol, e.g. "~", with a key combination, e.g. CTRL+FN+`?
You can use unicode entry as a stop gap---e.g. type LEFTSHIFT+LEFTCTL+u, followed by the unicode for tilde, 223c then ENTER. The unicode for backtick (actually an accent grave) is 0060.
I also did a search on AskUbuntu and found this old answer which is quite comprehensive One addition I might make is to use xdotool key asciitilde instead of the long sequence for the unicode.