Signal can be used with 12VPX in WEB or WEB for China mode. Unfortunately, they did make the procedure a little bit cumbersome. Please see below.
env https_proxy=http://127.0.0.1:16005 open -a "Signal"
set HTTPS_PROXY=http://127.0.0.1:16005
set NO_PROXY=localhost,127.0.0.1
start "" "%localappdata%\Programs\signal-desktop\Signal.exe"
If you're a more experienced user, you may want to put the commands in a batch- or script-file and create a shortcut to the script on your Desktop.