<div class="container">
  <div class="row">
    <div class="col p-4">
      <form class="post_password" action="'" method="post" style="max-width:20rem;">
			<div class="input-group mb-3 flex-nowrap">
				<div class="input-group-prepend">
					<span class="input-group-text"><i class="fa fa-key" aria-hidden="true"></i></span>
				</div>
				<input type="password" name="post_password" class="form-control" size="24" placeholder="パスワード" aria-label="パスワード" />
				<div class="input-group-append">
					<input type="submit" name="Submit" class="btn btn-secondary" value="送信" />
				</div>
			</div>
    </form>
    </div>
  </div>
</div>

External CSS

  1. https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css
  2. https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css

External JavaScript

  1. https://code.jquery.com/jquery-3.3.1.slim.min.js
  2. https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js
  3. https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js