如何在Wordpress的自定义邮件表单中使用插件添加验证码？

我正在使用插件，现在我想将其添加到我的自定义表单中：

<form>    
    <div class="span3 form_fields"><input type="text" name="fname" id="fname" placeholder="First Name (required)" class="span3"/></div>
    <div class="span3 form_fields"><input type="text" name="lname" id="lname" placeholder="Last Name (required)" class="span3"/></div>
    <div class="span3 form_fields"><input type="text" name="phone" id="phone" placeholder="Phone Number (required)" class="span3"/></div>
    <div class="span3 form_fields"><input type="text" name="company" id="company" placeholder="Company/ Institute Name (required)" class="span3"/></div>
    <div class="span3 form_fields"><input type="text" name="email" id="email" placeholder="Email (required)" class="span3"/></div>
    <div class="span3 form_fields"><select style="width:100%!important;" name="country" id="country">
            <option value="">Select country</option>
            <option value="India">India</option>
            <option value="USA">USA</option>
        </select>
    </div>
    <div class="span6 form_fields"><textarea placeholder="Message (required)" name="message" id="message" class="span6"/></textarea></div>
    <div class="clearfix"></div>

    //here i want to add captcha     

    <div><button type="button"  class="btn btn-inverse" id="submit" onclick="contactsales();">Submit</button></div>
</form>

选择国家
印度
美国
//这里我想添加验证码
提交

请帮我找出简单的解决办法

<form action="" method="post">
.... existing form data here ....  
    <?php
          // display the re-captcha box if re-captcha is installed:
          if(function_exists('display_recaptcha')){
               display_recaptcha();
          }
      ?>
</form> 

.... 此处的现有表单数据。。。。

我会试试的，谢谢……）