How do I add a sidebar to a Fullwidth page ?

Try this:

<?php
/**
 * Template Name: Full Width, no sidebar(s)
*/

get_header(); ?>

    <div id="main-fullwidth">

        <?php
            if (have_posts()) : while (have_posts()) : the_post();
                /**
                 * Find the post formatting for the pages in the post-page.php file
                 */
                get_template_part('post', 'page');

                if(comments_open( get_the_ID() ))  {
                    comments_template('', true);
                }
            endwhile;

            else :
                get_template_part('post', 'noresults');
            endif;
        ?>

    </div><!-- #main-fullwidth -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>

I get the sidebar, but it goes under the context. Can I in somehow move the context to the right, so the sidebar comes up on the left side of the context?

Can you show me a online demo?

https://pc-electronics.dk/?page_id=13