XmListSetAddMode(3) on Linux
The text 933cdc7ce0f4, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A List function that sets add mode in the list "XmListSetAddMode" "List functions" "XmListSetAddMode"