如何在垂直导航栏中创建下拉列表

我正在尝试在我的垂直导航栏中创建一个下拉列表。如果您能帮助我想出如何创建下拉列表的想法，我将不胜感激。

我尝试为我的项目研究和寻找不同的代码。以下是我在代码中尝试的一些内容。

我的 HTML 代码：

            <div class="menu-overlay"></div>
                <div class="side-menu-wrapper">
                   <a href="#" class="menu-close">×</a>
                   <ul>
                      <li>
                          <form class="example"action="action_page.php">
                              <input type="text" placeholder="Search.." 
                              name="search">
                             <button type="submit">
                                   <i class="fa fa-search"></i>
                             </button>
                           </form>
                       </li>
                       <li>
                          <a href="Index.html" target="_blank" 
                           rel="nofollow">
                                Home
                           </a>
                       </li>
                       <li >
                          <a href="#" target="_blank" rel="nofollow">
                               About
                          </a>
                         <ul>
                            <li href="#" target="_blank" rel="nofollow">
                                <a >Introduction</a>
                            </li>
                            <li href="#" target="_blank" rel="nofollow"> 
                                <a >Mission</a>
                            </li>
                            <li href="#" target="_blank" rel="nofollow"> 
                                 <a >Vision</a>
                            </li>
                         </ul>
                     </li>
                     <li>
                        <a href="#" target="_blank" rel="nofollow">
                            Facebook
                        </a>
                     </li>
                     <li>
                       <a href="#" target="_blank" 
                       rel="nofollow">
                            Flickr
                        </a>
                   </li>
                </ul>
           </div>

我的 CSS 代码：

.side-menu-wrapper {
   background: rgba(0,114,187,0.7);
   padding: 40px 0 0 40px;
   position: fixed;
   top: 0;
   right: 0; 
   height: 100%;
   z-index: 2;
   transition: 0.5s;
   width: 250px;
   font: 20px "Courier New", Courier, monospace;
   box-sizing: border-box;
 }

.side-menu-wrapper > ul{
   list-style:none;
   padding:0;
   margin:0;
   overflow-y: auto;
   height:95%;
}

.side-menu-wrapper > ul > li > a {
    display: block;
    border-bottom: 1px solid black;
    padding: 6px 4px 6px 4px;
    color: white;
    transition: 0.3s;
    text-decoration: none;
  }

.side-menu-wrapper > a.menu-close { 
   padding: 8px 0 4px 23px;
   color: white;
   display: block;
   margin: -30px 0 -10px -20px;
   font-size: 35px;    
   text-decoration: none;
 }

.menu-overlay { 
  height: 100%;
  width: 0;
  position: fixed;
  z-index: 1;
  top: 0;
  left: 0;
  background-color: rgba(0,0,0,.7);
  overflow-y: auto;
  overflow-x: hidden;
  text-align: center;
  opacity: 0;
  transition: opacity 1s;
}

结果：

顺便说一句