I am using Editable dropdownlist and bind with database.
I want a watermark like Textbox on my dropdownlist.
  <!-- Editable Dropdown -->
  <link href="https://code.jquery.com/ui/1.11.4/themes/smoothness/jquery-ui.css" rel="stylesheet" />
  <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
  <script src="https://code.jquery.com/ui/1.11.4/jquery-ui.js"></script>
  <link href="https://jqueryui.com/resources/demos/style.css" rel="stylesheet" />
      <style>
         
    .custom-combobox {
      position: relative;
      display: inline-block;
    }
    .custom-combobox-toggle {
      position: absolute;
      top: 0;
      bottom: 0;
      margin-left: -1px;
      padding: 0;
      width: 50%;
    }
    .custom-combobox-input {
      margin: 0;
      padding: 5px 10px;
    }
    .ui-state-default,
    .ui-widget-content .ui-state-default,
    .ui-widget-header .ui-state-default {
      background: #fbfbfb;
  border: 1px solid #cfcdcd;
  color: #87929c;
  float: left;
  font-family: 'Open Sans', sans-serif;
  font-size: 14px;
  font-weight: 600;
  margin: 0 10px 0 0;
  padding: 12px 10px 12px 50px;
  width: 100%;
  border-radius:5px;
    }
    /* Corner radius */
    .ui-corner-all,
    .ui-corner-top,
    .ui-corner-left,
    .ui-corner-tl {
      border-top-left-radius: 0px !important;
    }
    .ui-corner-all,
    .ui-corner-top,
    .ui-corner-right,
    .ui-corner-tr {
      border-top-right-radius: 0px !important;
    }
    .ui-corner-all,
    .ui-corner-bottom,
    .ui-corner-left,
    .ui-corner-bl {
      border-bottom-left-radius: 0px !important;
    }
    .ui-corner-all,
    .ui-corner-bottom,
    .ui-corner-right,
    .ui-corner-br {
      border-bottom-right-radius: 0px !important;
    }
  </style>
      <script>
    (function($) {
      $.widget("custom.combobox", {
        _create: function() {
          this.wrapper = $("<span>")
            .addClass("custom-combobox")
            .insertAfter(this.element);
          this.element.hide();
          this._createAutocomplete();
          this._createShowAllButton();
        },
        _createAutocomplete: function() {
          var selected = this.element.children(":selected"),
            value = selected.val() ? selected.text() : "";
          this.input = $("<input>")
            .appendTo(this.wrapper)
            .val(value)
            .attr("title", "")
            .addClass("custom-combobox-input ui-widget ui-widget-content ui-state-default ui-corner-left")
            .autocomplete({
              delay: 0,
              minLength: 0,
              source: $.proxy(this, "_source")
            })
            .tooltip({
              tooltipClass: "ui-state-highlight"
            });
          this._on(this.input, {
            autocompleteselect: function(event, ui) {
              ui.item.option.selected = true;
              this._trigger("select", event, {
                item: ui.item.option
              });
            },
            autocompletechange: "_removeIfInvalid"
          });
        },
        
        _source: function(request, response) {
          var matcher = new RegExp($.ui.autocomplete.escapeRegex(request.term), "i");
          response(this.element.children("option").map(function() {
            var text = $(this).text();
            if (this.value && (!request.term || matcher.test(text)))
              return {
                label: text,
                value: text,
                option: this
              };
          }));
        },
        _removeIfInvalid: function(event, ui) {
          // Selected an item, nothing to do
          if (ui.item) {
            return;
          }
          // Search for a match (case-insensitive)
          var value = this.input.val(),
            valueLowerCase = value.toLowerCase(),
            valid = false;
          this.element.children("option").each(function() {
            if ($(this).text().toLowerCase() === valueLowerCase) {
              this.selected = valid = true;
              return false;
            }
          });
          // Found a match, nothing to do
          if (valid) {
            return;
          }
          // Remove invalid value
          this.input
            .val("")
            .attr("title", value + " didn't match any item")
            .tooltip("open");
          this.element.val("");
          this._delay(function() {
            this.input.tooltip("close").attr("title", "");
          }, 2500);
          this.input.autocomplete("instance").term = "";
        },
        _destroy: function() {
          this.wrapper.remove();
          this.element.show();
        }
      });
    })(jQuery);
    $("#combobox").focus(function () { $(this).select(); });
    $(function () {
       
      $("#combobox").combobox();
      $("#toggle").click(function() {
        $("#combobox").toggle();
      });
    });
  </script>