I think the essential code part is; (I try to omitted non-essential code)

<?php   
if(isset($_POST['submit-coupon']) ) {
    require get_template_directory() . '/includes/mediaUpload.php';
    
    // error handling codes
    
    if($_FILES['coupon_image']['error'] != 0) {
        $errors[] = __('Купон-ийн зураг оруулна уу', 'Teo');
    }
    if(count($errors) == 0) {
        $allowed_tags = wp_kses_allowed_html( 'post' );
        $title = esc_attr($_POST['coupon_title']);
        $description = wp_kses($_POST['coupon_description'], $allowed_tags);
        if(is_user_logged_in() ) { 
            $current_user = wp_get_current_user();
            $user_id = $current_user->ID;
        }
        else {
            $user_id = 1;
        }
        $post = array(
        'comment_status' => 'close', /* added by AB, open */
        'ping_status'    => 'close', /* added by AB, open */
        'post_author'    => $user_id,
        'post_content'   => $description,
        'post_status'    => 'publish', /* added by AB, pending */
        'post_title'     => $title,
        'post_type'      => 'coupons',
        );  
        $post_id = wp_insert_post($post);
        if($post_id == 0) {
            $errors[] = __("The listing couldn't be added to the database, try again.", "Teo");
        }
        else {
            $cat = (array)$cat;
            wp_set_post_terms($post_id, $cat, 'category'); //setting the correct category
            $tmp = new MediaUpload;
            if($_FILES['coupon_image']['error'] == 0) {
                $featured_image = $tmp->saveUpload( 'coupon_image' );
                add_post_meta($post_id, '_thumbnail_id', $featured_image['attachment_id']);
            }
            //setting custom fields info
            if($discount != '') {
                add_post_meta($post_id, '_single_discount', $discount);
            $ok=1;
        }
    }
    header('Location: http://coupon.mn');     
}?>


<div class="text-content frontend-submit">
<?php if(isset($errors) && count($errors) > 0) { ?>
    <div class="large-12 column alert alert-danger">
        <?php foreach($errors as $error) echo $error . '<br />'; ?>
    </div>
<?php } ?>
<?php if(isset($ok) ) { ?>
    <div class="large-12 column alert alert-success">
        <?php _e('Your post is added!', 'Couponize');?>
    </div>
<?php } ?>
<form enctype="multipart/form-data" action="<?php echo(get_permalink($post->ID)); ?>" method="post">
    
    
    // MORE INPUT FORM
    
    <!-- SUBMIT BUTTON -->
    <div class="cf"></div> <br />
    <div class="large-12 column">
        <input class="input button primary red, large-3" type="submit" name="submit-coupon" value="+ купон нэмэх" />
            

I'm using the following code to redirect the page, in order to prevent re-submission of the form. And it does fine job doing redirection to the homepage.

But I want redirection to uploaded respective post after form submission from front-end.

if(isset($_POST['submit-coupon']) ) {
    // other codes
     header('Location: http://coupon.mn');     
}

I'm using the following code to redirect the page, in order to prevent re-submission of the form. And it does fine job doing redirection to the homepage.

And my website is based on the WordPress, therefore once the (WordPress) post's form is uploaded, I want redirection to the uploaded respective post after form submission (form is uploaded from the front-end)

Therefore what kind of code, do I need to put in the header() function.

if(isset($_POST['submit-coupon']) ) {
    // other codes
     header('Location: http://coupon.mn');     
}