Icon in bootstrap 4 inside the input

span{
  position: absolute;
  margin-left: 5px;
  height: 25px;
  display: flex;
  align-items: center;
}
input{
  padding-left: 25px;
  height: 20px;
}

<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>

<div class="input-group">
  <span>
    <i class="fa fa-user-circle-o" aria-hidden="true"></i>
  </span>
  <input type="text" class="form-control" placeholder="Username" />
</div>