Form Validation - IF field is blank THEN automatically selection option

I need help to automatically select an option to submit with a form: When the 'form-email' field is blank i want it to select 'option 1' and, When the field is not blank i want it to select 'option 2'.

Here's my form code

<form method="post" onsubmit="return validate-category(this)" action="tdomf-form-post.php" id='tdomf_form1' name='tdomf_form1' class='tdomf_form'> 

    <textarea title="Post Title" name="content-title-tf" id="form-content" >Say it...</textarea>

    <input type="text" value="" name="content-text-ta" id="form-email"/>

    <select name='categories' class='form-category' type="hidden"> 
  开发者_如何学编程      <option value="3" type="hidden">Anonymous</option> 
        <option value="4" type="hidden" selected="selected">Competition</option> 
    </select>

    <input type="submit" value="Say it!" name="tdomf_form1_send" id="form-submit"/>

</form>

I have an idea that the javascript would go something like this, but can't find what the code is to change the value.

<script type="text/javascript">
function validate-category(field)
{
with (field)
  {
  if (value==null||value=="")
    {
    select category 1
    }
  else
    {
    select category 2
    return true;
    }
  }
}
</script>

Any help on this would be great.

Thanks in advance.

Give the selectbox an ID:

<select name='categories' class='form-category' type="hidden" id="categories"> 
    <option value="3" type="hidden">Anonymous</option> 
    <option value="4" type="hidden" selected="selected">Competition</option> 
</select>

Then use this code:

<script type="text/javascript">
function validate-category(field)
{
with (field)
  {
  if (value==null||value=="")
    {
    document.getElementById("categories").value = "3";
    }
  else
    {
    document.getElementById("categories").value = "4";

    }
  }
 return true;
}
</script>