<form id="side-arch">
     <select name="archive-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
          <option value=""><?php echo esc_attr( __( 'Select Month' ) ); ?></option>
          <?php wp_get_archives( array( 'type' => 'monthly', 'format' => 'option', 'show_post_count' => 1 ) ); ?>
     </select>
     <input type="submit" value="Go" id="submit" />
</form>

<form id="side-arch" action="<?php echo esc_url( home_url( '/' ) ); ?>" method="get">
     <select name="archive-dropdown">
          <option value=""><?php echo esc_attr( __( 'Select Month' ) ); ?></option>
          <?php wp_get_archives( array( 'type' => 'monthly', 'format' => 'option', 'show_post_count' => 1 ) ); ?>
     </select>
     <input type="submit" value="Go" id="submit" />
</form>