Sticky form dropdown

I have the code below in a form. The select box is used to select a category or if the last option is selected it displays an input to add a new category.

I need the select box to be sticky (if they hit send but but has errors it keeps the value selected.

Thanks for any help!

<select name='categories-1' id='categories-1' class='categories-1' size='1' onchange="displayNewCat(this);">
   <option class="green" value="">Select/Add category</option>
   <?php
	$categories=  get_categories('child_of=4');
	foreach ($categories as $cat) {
	   $option = '<option value="'.$cat->cat_name.'">&nbsp;&nbsp;&nbsp;&nbsp;';
	   $option .= $cat->cat_name;
	   $option .= ' ('.$cat->category_count.')';
	   $option .= '</option>';
	   echo $option;
	}
   ?>
   <option disabled="disabled" class="green">Add New</option>
   <option value="new">&nbsp;&nbsp;&nbsp;&nbsp;Click Here to Add New </option>
</select>