XmTextSetAddMode(3) on Linux
The text f9871d2c8244, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A Text function that sets the state of Add mode "XmTextSetAddMode" "Text functions" "XmTextSetAddMode"