Man pages matching name xmvacreatemanagedmainwindow in section 3:

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