XmSetProtocolHooks(3) on Linux
The text 16c659f887df, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A VendorShell function that allows preactions and postactions to be executed when a protocol message is received from MWM "XmSetProtocolHooks" "VendorShell functions" "XmSetProtocolHooks" "protocols"