小编Has*_*san的帖子

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
<form>
  <div class="col-lg-10 mb-3">
    <div class="input-group">
      <input type="text" class="form-control rounded-0" id="validationDefaultUsername" placeholder="Username" aria-describedby="inputGroupPrepend2" required>
      <div class="input-group-prepend">
        <input type="submit" vlaue="submit" class="btn btn-primary btn-sm rounded-0" id="inputGroupPrepend2">
      </div>
    </div>
  </div>
</form>