<form class="form-asd" role="form">
    <h2 class="form-signin-heading">login</h2><hr />
    <label class="control-label"  for="username">Username</label>
    <input class="form-control" type="email"  required="" placeholder="username"data-error="enter valid username"></input>
    <label class="control-label"  for="username">password</label>
    <input class="form-control" type="password"  required=" " placeholder="Password"></input>
    <label class="checkBox"></label>
    <button class="btn btn-lg btn-primary " type="submit">submit</button>
</form>

我们如何更改这个pop请求字段的“请填写这个字段”的默认消息到“请输入用户名”