XmAddWMProtocolCallback(3) on Linux
The text dead9a954c7b, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A VendorShell convenience interface that adds client callbacks for a protocol "XmAddWMProtocol\%Callback" "VendorShell functions" "XmAddWMProtocol\%Callback" "protocols"