Si entendi bien. Tenes que poner el siguiente codigo el en metodo lookup del cambo en el design….
public void lookup() {
FormStringcontrol formStringControl;
SysTableLookup sysTableLookup ;
Query query = new Query();
QueryBuildDataSource queryBuildDataSource = query.addDataSource(tablenum(TablaCatalogo));
QueryBuildRange queryBuildRange = queryBuildDataSource.addRange(fieldnum(TablaCatalogo,field1));
;
formStringControl = StringEdit1; // nombre del combo
sysTableLookup = SysTableLookup::newParameters(tablenum(TablaCatalogo), formStringControl);
queryBuildRange.value(queryValue(“0001”));
sysTableLookup.addLookupfield(fieldnum(TablaCatalogo, field1));
sysTableLookup.addLookupfield(fieldnum(TablaCatalogo, field2));
sysTableLookup.parmQuery(query);
sysTableLookup.performFormLookup();
}