HTML

center vertically and horizontally bootstrap 4

<div class="container h-100">
  <div class="row h-100 justify-content-center align-items-center">
    <form class="col-12">
      <div class="form-group">
        <label for="formGroupExampleInput">Example label</label>
        <input type="text" class="form-control" id="formGroupExampleInput" placeholder="Example input">
      </div>
      <div class="form-group">
        <label for="formGroupExampleInput2">Another label</label>
        <input type="text" class="form-control" id="formGroupExampleInput2" placeholder="Another input">
      </div>
    </form>   
  </div>
</div>

bootstrap center div vertically and horizontally

 <div class="container h-100">
  <div class="d-flex justify-content-md-center align-items-center vh-100">
   <p>Your Content</p>
  </div>     
 </div>