Man pages matching name xmvacreatemanagedmessagebox in section 3:

XmVaCreateManagedMessageBox.3.en	debian 10 / libmotif-dev
XmVaCreateManagedMessageBox.3.en	debian 11 / libmotif-dev
XmVaCreateManagedMessageBox.3.en	debian 12 / libmotif-dev