dropdown forms in bootstrap

Bootstrap’s .button styles can be applied to other elements, such as <label>s, to provide checkbox
or radio style button toggling. Add data-toggle="buttons" to a .btn-group 
containing those modified buttons to enable their toggling behavior via 
JavaScript and add .btn-group-toggle to style the <input>s within your buttons.
Note that you can create single input-powered buttons or groups of them.

The checked state for these buttons is only updated via click event on the 
button. If you use another method to update the input—e.g., with 
<input type="reset"> or by manually applying the input’s checked 
property—you’ll need to toggle .active on the <label> manually.

Note that pre-checked buttons require you to manually add the .active class to 
the input’s <label>.

<div class="btn-group-toggle" data-toggle="buttons">
  <label class="btn btn-secondary active">
    <input type="checkbox" checked> Checked
  </label>
</div>

<div class="col-lg-6 col-md-6 col-12">
  <div class="input-group mb-3">
    <div class="input-group-prepend">
      <select class="form-control select2bs4" name="country_code" id="country_code" style="width: 100%">
        <option value="+91">+91</option>
        <option value="+351">+351</option>
        <option value="+1">+1</option>
      </select>
    </div>
    	<input type="text" name="user" class="form-control" placeholder="Email Or Mobile Number">
    </div>
</div>

          <li class="dropdown">
         <a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown            

         <b class="caret"></b></a>
         <ul class="dropdown-menu">
           <li><a href="#"><form role="form">
<div class="form-group">
  <label for="sel1">Select list (select one):</label>
  <select class="form-control" id="sel1">
    <option>1</option>
    <option>2</option>
    <option>3</option>
    <option>4</option>
  </select>
  <br>
  <label for="sel2">Mutiple select list (hold shift to select more than one):</label>
  <select multiple class="form-control" id="sel2">
    <option>1</option>
    <option>2</option>
    <option>3</option>
    <option>4</option>
    <option>5</option>
  </select>
</div>