$('#autocomplete').autocomplete({
            source: 'http://catalog.max/search/',
            minLength: 1,
            select: function( event, ui ){
                console.log(ui.item.value);
            }
        });

<form action="<?=PATH?>search/" method="get">
            <ul class="search">
                <li>
                    <input type="text" id="autocomplete" class="search" name="search" />
                </li>
                <li>
                    <input type="submit" class="search-go" name="go-search" value="поиск"  />
                </li>
            </ul>
        </form>