Use output of wp_dropdown_categories($args)

Please be gentle, I’m new to all this coding!
I have finally worked out how to get a frontend dropdown field allowing me to select from EDD categories (I got this from the Codex):

<div class="gallery-row">
  <li id="categories">
    <h2><?php _e( 'Categories:' ); ?></h2>
    <form id="category-select" class="category-select" action="<?php echo esc_url( home_url( '/' ) ); ?>" method="get">
      <?php $args = array(
        'taxonomy' => 'download_category', // EDD custom post type Taxonomy
        'order'    => 'ASC' // Order of the list
      );
      wp_dropdown_categories($args); ?>
      <input type="submit" name="submit" value="view" />
    </form>
  </li>
</div>

This is my original code,’selecttaxonomy’ is a ACF taxonomy field (and only appears on the backend):

<div class="content clearfix"><div class="gallery-row2">
  <?php
  $current_page = get_query_var('paged'); // Retrieving the data
  $per_page = get_option('posts_per_page');
  $offset = $current_page > 0 ? $per_page * ($current_page -1) : 0;
  $taxval = get_field('selecttaxonmy');

  $product_args = array(
    'post_type'         => 'download',
    'posts_per_page'    => $per_page,
    'offset'            => $offset,
    'download_category' => $taxval,
  );
  $products = new WP_Query($product_args);
  ?>

  <?php if ($products->have_posts()) : $i = 1; ?> //and off into the loop.....

What I can’t figure out is how to replace ‘selecttaxonomy’ with the output of the dropdown – help please.
The page I’m working on is: https://ageingdj.com/0anotheredd-test/ (ignore the styling please)