Refer a friend by WPGens - Woocommerce Subscription discount

functions.php

<?php

add_filter('wcs_new_order_created','gens_renewal_order_created', 10, 2 );
function gens_renewal_order_created($order, $subscription){  

	$_user_id = $order->get_user_id();
	$user_info = get_userdata($_user_id);
	$user_email = $user_info->user_email;
	$args = array(
		'posts_per_page'   => -1,
	    'post_type'        => 'shop_coupon',
	    'post_status'      => 'publish',
		'meta_query' => array(
			'relation' => 'AND',
		    array(
			  'key' => 'customer_email',
			  'value' => $user_email,
              'compare' => 'LIKE'
		    ),
		    array(
		    	'relation' => 'OR',
		    	array(
			    	'key' => 'usage_count',
			    	'value' => '0',
			    	'compare' => 'LIKE'
			    ),
		    	array(
			    	'key' => 'usage_count',
			    	'compare' => 'NOT EXISTS'
		    	)
		    )
		),
	);
	$coupons = get_posts( $args );
	
	if(empty($coupons)) {
		return $order;
	}
	
	$coupons_obj = new WC_Coupon($coupons[0]->ID);
	$amount = $coupons_obj->get_amount();
	$type = $coupons_obj->get_discount_type(); // fixed_cart or percent
	$coupons_amount = $coupons_obj->set_usage_count(1);
	$coupons_obj->save();

	if( $type == "percent") {
		$discount = $order->get_total() * ( $amount / 100);
	} else {
		$discount = $amount;
	}

	$item = new WC_Order_Item_Fee();
	$item->set_props( array(
	    'name'      => "Referral applied",
	    'tax_class' => NULL,
	    'total'     => -$discount,
	    'total_tax' => 0,
	    'order_id'  => $order->get_id(),
	) );
	$item->save();
	$order->add_item( $item );
	$order->update_taxes();
	$order->calculate_totals( true );

    return $order;

}

From class-wc-order-item-fee.php

• Fee is an amount of money charged for a particular piece of work
• or for a particular right or service, and not supposed to be negative.

I checked and for now the total may be negative as used here. But we do not know if this will be accepted in the future.

So right now I am using a
$newOrder->set_total( $newOrder->get_total() - $discountAmount );
followed by
$newOrder->add_order_note( __( 'Coupon applied', 'textdomain' ) );