XmTextSetSelection(3) on Linux
The text 724d8806b14c, 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 primary selection of the text "XmTextSetSelection" "Text functions" "XmTextSetSelection"