var expanded = false;

function showCheckboxes() {
  var checkboxes = document.getElementById("checkboxes");
  if (!expanded) {
    checkboxes.style.display = "block";
    expanded = true;
  } else {
    checkboxes.style.display = "none";
    expanded = false;
  }
}

.multiselect {
  width: 200px;
}

.selectBox {
  position: relative;
}

.selectBox select {
  width: 100%;
  font-weight: bold;
}

.overSelect {
  position: absolute;
  left: 0;
  right: 0;
  top: 0;
  bottom: 0;
}

#checkboxes {
  display: none;
  border: 1px #dadada solid;
}

#checkboxes label {
  display: block;
}

#checkboxes label:hover {
  background-color: #1e90ff;
}

<form>
  <div class="multiselect">
    <div class="selectBox" onclick="showCheckboxes()">
      <select>
        <option>Select an option</option>
      </select>
      <div class="overSelect"></div>
    </div>
    <div id="checkboxes">
      <label for="one">
        <input type="checkbox" id="one" />First checkbox</label>
      <label for="two">
        <input type="checkbox" id="two" />Second checkbox</label>
      <label for="three">
        <input type="checkbox" id="three" />Third checkbox</label>
    </div>
  </div>
</form>

$(function() {

  $('#chkveg').multiselect({
    includeSelectAllOption: true
  });

  $('#btnget').click(function() {
    alert($('#chkveg').val());
  });
});

.multiselect-container>li>a>label {
  padding: 4px 20px 3px 20px;
}

<link href="https://unpkg.com/bootstrap@3.3.2/dist/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://unpkg.com/jquery@3.3.1/dist/jquery.min.js"></script>
<script src="https://unpkg.com/bootstrap@3.3.2/dist/js/bootstrap.min.js"></script>
<script src="https://unpkg.com/bootstrap-multiselect@0.9.13/dist/js/bootstrap-multiselect.js"></script>
<link href="https://unpkg.com/bootstrap-multiselect@0.9.13/dist/css/bootstrap-multiselect.css" rel="stylesheet"/>

<form id="form1">
  <div style="padding:20px">

    <select id="chkveg" multiple="multiple">
      <option value="cheese">Cheese</option>
      <option value="tomatoes">Tomatoes</option>
      <option value="mozarella">Mozzarella</option>
      <option value="mushrooms">Mushrooms</option>
      <option value="pepperoni">Pepperoni</option>
      <option value="onions">Onions</option>
    </select>
    
    <br /><br />

    <input type="button" id="btnget" value="Get Selected Values" />
  </div>
</form>

$(".multiple_select").mousedown(function(e) {
    if (e.target.tagName == "OPTION") 
    {
      return; //don't close dropdown if i select option
    }
    $(this).toggleClass('multiple_select_active'); //close dropdown if click inside <select> box
});
$(".multiple_select").on('blur', function(e) {
    $(this).removeClass('multiple_select_active'); //close dropdown if click outside <select>
});
	
$('.multiple_select option').mousedown(function(e) { //no ctrl to select multiple
    e.preventDefault(); 
    $(this).prop('selected', $(this).prop('selected') ? false : true); //set selected options on click
    $(this).parent().change(); //trigger change event
});

	
	$("#myFilter").on('change', function() {
      var selected = $("#myFilter").val().toString(); //here I get all options and convert to string
      var document_style = document.documentElement.style;
      if(selected !== "")
        document_style.setProperty('--text', "'Selected: "+selected+"'");
      else
        document_style.setProperty('--text', "'Select values'");
	});

:root
{
	--text: "Select values";
}
.multiple_select
{
	height: 18px;
	width: 90%;
	overflow: hidden;
	-webkit-appearance: menulist;
	position: relative;
}
.multiple_select::before
{
	content: var(--text);
	display: block;
  margin-left: 5px;
  margin-bottom: 2px;
}
.multiple_select_active
{
	overflow: visible !important;
}
.multiple_select option
{
	display: none;
    height: 18px;
	background-color: white;
}
.multiple_select_active option
{
	display: block;
}

.multiple_select option::before {
  content: "\2610";
}
.multiple_select option:checked::before {
  content: "\2611";
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select id="myFilter" class="multiple_select" multiple>
  <option>A</option>
  <option>B</option>
  <option>C</option>
  <option>D</option>
  <option>E</option>
</select>