Man pages matching name xmvacreatemanagedbuttonbox in section 3:

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