To start using  the wpDiscuz shortcode, just put the following code in the active theme functions.php file:

function my_wpdiscuz_shortcode() {
    $html = "";
    if (file_exists(ABSPATH . "wp-content/plugins/wpdiscuz/themes/default/comment-form.php")) {
        ob_start();
        include ABSPATH . "wp-content/plugins/wpdiscuz/themes/default/comment-form.php";
        $html = ob_get_clean();
    }
    return $html;
}
add_shortcode("wpdiscuz_comments", "my_wpdiscuz_shortcode");

Here is an instruction on how to easily add custom code in WordPress (without Breaking Your Site): https://www.wpbeginner.com/plugins/how-to-easily-add-custom-code-in-wordpress-without-breaking-your-site/

Then, you can use the shortcode in the post/page content wherever you want. 

Besides adding the shortcode, you'd also need to enable the comment form for the current post type. 

That is: the shortcode can be used if the post type supports comments.