Man pages matching name xmtextfieldgetsubstringwcs in section 3:

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