i'm a newbie with tampermonkey and i want to auto fill a form but has no id ...i already tried some codes but doesnt work

source

```
<input type="text" name="user_email" class="form-control-input" value="" placeholder="Enter your Email" onselectstart="return false" onpaste="return false;" oncopy="return false" oncut="return false" ondrag="return false" ondrop="return false" autocomplete="off">

```

code already tested

```
  var mail = "test@gmail.com";
  document.getElementsByClassName('form-control-input').value = mail;

```