Drop Down Filter with CPT as Options for Search Form

use for search this form with custom post type list

<form role="search" method="get" id="searchform" action="<?php echo home_url( "https://wordpress.stackexchange.com/" ); ?>">
        <input type="text" name="s" id="s" value="Enter keywords ..." onfocus="if(this.value==this.defaultValue)this.value="";" onblur="if(this.value=='')this.value=this.defaultValue;"/><br />
    <?php $types = get_post_types(); ?>

        <select name="posttype">
    <?php foreach ( $types as $type ) {?>
          <option name="<?php echo $type->singular_label ; ?>"
     value="<?php echo $type->singular_label ; ?>"><?php echo $type->singular_label ; ?></option>
    <?php }?>
    </select>
    <input type="submit" id="searchsubmit" value="Search Help" />
</form>