How to Create Checkbox Inside Dropdown?

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>

window.onload = (event) => {
  initMultiselect();
};

function initMultiselect() {
  checkboxStatusChange();

  document.addEventListener("click", function(evt) {
    var flyoutElement = document.getElementById('myMultiselect'),
      targetElement = evt.target; // clicked element

    do {
      if (targetElement == flyoutElement) {
        // This is a click inside. Do nothing, just return.
        //console.log('click inside');
        return;
      }

      // Go up the DOM
      targetElement = targetElement.parentNode;
    } while (targetElement);

    // This is a click outside.
    toggleCheckboxArea(true);
    //console.log('click outside');
  });
}

function checkboxStatusChange() {
  var multiselect = document.getElementById("mySelectLabel");
  var multiselectOption = multiselect.getElementsByTagName('option')[0];

  var values = [];
  var checkboxes = document.getElementById("mySelectOptions");
  var checkedCheckboxes = checkboxes.querySelectorAll('input[type=checkbox]:checked');

  for (const item of checkedCheckboxes) {
    var checkboxValue = item.getAttribute('value');
    values.push(checkboxValue);
  }

  var dropdownValue = "Nothing is selected";
  if (values.length > 0) {
    dropdownValue = values.join(', ');
  }

  multiselectOption.innerText = dropdownValue;
}

function toggleCheckboxArea(onlyHide = false) {
  var checkboxes = document.getElementById("mySelectOptions");
  var displayValue = checkboxes.style.display;

  if (displayValue != "block") {
    if (onlyHide == false) {
      checkboxes.style.display = "block";
    }
  } else {
    checkboxes.style.display = "none";
  }
}

.multiselect {
  width: 100%;
}

.selectBox {
  position: relative;
}

.selectBox select {
  width: 100%;
}

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

#mySelectOptions {
  display: none;
  border: 0.5px #7c7c7c solid;
  background-color: #ffffff;
  max-height: 150px;
  overflow-y: scroll;
}

#mySelectOptions label {
  display: block;
  font-weight: normal;
  display: block;
  white-space: nowrap;
  min-height: 1.2em;
  background-color: #ffffff00;
  padding: 0 2.25rem 0 .75rem;
  /* padding: .375rem 2.25rem .375rem .75rem; */
}

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

<script src=""></script>
<link href="" rel="stylesheet"/>

<div class="container-fluid">


  <div class="form-group col-sm-8">
    <label for="dur">BS original select</label>
    <select id="dur" class="form-select">
      <option value="12" selected>One Year</option>
      <option value="24">Two Year</option>
      <option value="36">Three Year</option>
      <option value="48">Four year</option>
      <option value="60">Five Year</option>
    </select>
  </div>

  <div class="form-group col-sm-8">
    <label for="myMultiselect">BS custom multiselect</label>
    <div id="myMultiselect" class="multiselect">
      <div id="mySelectLabel" class="selectBox" onclick="toggleCheckboxArea()">
        <select class="form-select">
          <option>somevalue</option>
        </select>
        <div class="overSelect"></div>
      </div>
      <div id="mySelectOptions">
        <label for="one"><input type="checkbox" id="one" onchange="checkboxStatusChange()" value="one" /> First checkbox</label>
        <label for="two"><input type="checkbox" id="two" onchange="checkboxStatusChange()" value="two" /> Second checkbox</label>
        <label for="three"><input type="checkbox" id="three" onchange="checkboxStatusChange()" value="three" /> Third checkbox</label>
        <label for="four"><input type="checkbox" id="four" onchange="checkboxStatusChange()" value="four" /> Third checkbox</label>
        <label for="five"><input type="checkbox" id="five" onchange="checkboxStatusChange()" value="five" /> First checkbox</label>
        <label for="six"><input type="checkbox" id="six" onchange="checkboxStatusChange()" value="six" /> Second checkbox</label>
        <label for="seven"><input type="checkbox" id="seven" onchange="checkboxStatusChange()" value="seven" /> Third checkbox</label>
        <label for="eight"><input type="checkbox" id="eight" onchange="checkboxStatusChange()" value="eight" /> First checkbox</label>
        <label for="nine"><input type="checkbox" id="nine" onchange="checkboxStatusChange()" value="nine" /> Second checkbox</label>
        <label for="ten"><input type="checkbox" id="ten" onchange="checkboxStatusChange()" value="ten" /> Third checkbox</label>
      </div>
    </div>
  </div>

</div>