custom-filter.js

function filterList(componentName, label, filter) {
    let component = PF(componentName),
        itemLabel = label.toLocaleLowerCase(),
        item = $(component.items)
            .filter(function () {
                return $(this).data('label').toString().toLocaleLowerCase() === itemLabel;
            })
            .first();

    if (item === undefined) {
        return false;
    }

    let value = $(item.children().last()).html().toString().trim().toLocaleLowerCase();
    return value.startsWith(filter);
}

function filterApplicant(label, filter) {
    return filterList('widget_mainTab_applicantForm_countryCodeApplicant', label, filter);
}

<p:selectOneMenu
    filter="true"
    filterFunction="filterApplicant">