Add the following Custom CSS to the Public Custom CSS field under Theme Settings >> Custom CSS.
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//Kupon Kodu Otomatik Açılır Hale Getirme Kodu//
.woocommerce-checkout .checkout_coupon.woocommerce-form-coupon {
    display: block !important;
}

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In the functions.php, add the following code:
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// Remove default coupon field
remove_action( 'woocommerce_before_checkout_form', 'woocommerce_checkout_coupon_form', 10 );

// Add a custom coupon field before checkout payment section
add_action( 'woocommerce_review_order_before_payment', 'woocommerce_checkout_coupon_form_custom' );
function woocommerce_checkout_coupon_form_custom() {
    echo '<div class="coupon-form" style="margin-bottom:20px;">
        <p>' . __("İNDİRİM KODU GİR") . '</p>
        <p class="form-row form-row-first woocommerce-validated">
            <input type="text" name="coupon_code" class="input-text" placeholder="' . __("Kodunuzu Giriniz") . '" id="coupon_code" value="">
        </p>
        <p class="form-row form-row-last">
            <button type="button" class="button" name="apply_coupon" value="' . __("Apply coupon") . '">' . __("UYGULA") . '</button>
        </p>
        <div class="clear"></div>
    </div>';
}

// jQuery - Send Ajax request
add_action( 'wp_footer', 'custom_checkout_jquery_script' );
function custom_checkout_jquery_script() {
    if ( is_checkout() && ! is_wc_endpoint_url() ) :
    ?>
    <script type="text/javascript">
    jQuery( function($){
        if (typeof wc_checkout_params === 'undefined')
            return false;

        var couponCode = '';

        $('input[name="coupon_code"]').on( 'input change', function(){
            couponCode = $(this).val();
        });

        $('button[name="apply_coupon"]').on( 'click', function(){
            $.ajax({
                type: 'POST',
                url: wc_checkout_params.ajax_url,
                data: {
                    'action': 'apply_checkout_coupon',
                    'coupon_code': couponCode,
                },
                success: function (response) {
                    $(document.body).trigger("update_checkout"); // Refresh checkout
                    $('.woocommerce-error,.woocommerce-message').remove(); // Remove other notices
                    $('input[name="coupon_code"]').val(''); // Empty coupon code input field
                    $('form.checkout').before(response); // Display notices
                    // console.log(response); // Uncomment for testing
                }
            });
        });
    });
    </script>
    <?php
    endif;
}

// Ajax receiver function
add_action( 'wp_ajax_apply_checkout_coupon', 'apply_checkout_coupon_ajax_receiver' );
add_action( 'wp_ajax_nopriv_apply_checkout_coupon', 'apply_checkout_coupon_ajax_receiver' );
function apply_checkout_coupon_ajax_receiver() {
    if ( isset($_POST['coupon_code']) && ! empty($_POST['coupon_code']) ) {
        WC()->cart->add_discount( wc_format_coupon_code( wp_unslash( $_POST['coupon_code'] ) ) ); // phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotSanitized
    } else {
        wc_add_notice( WC_Coupon::get_generic_coupon_error( WC_Coupon::E_WC_COUPON_PLEASE_ENTER ), 'error' );
    }
    wc_print_notices();
    wp_die();
}