Custom search criteria can be set u in SETTINGS -> BOOKING PAGE ->MULTIPLE PROPERTIES-> SEARCH CRITERIA. The context help (? icons) and the page help give detailed information.
To always show the search criteria without opening the "Advances Search" link go to SETTINGS->BOOKING PAGE->MULTIPLE PROPERTIES->DEVELOPERS and add
<script>
$( document ).ready(function() {
$("#collapseAdvancedSearch").show();
});
</script>
in "Advanced HTML Settings".