Automatically issue proforma invoice when order is on-hold (newly created).

generate_proforma.php

<?php
/**
 * Automatically issue proforma invoice when order is on-hold (newly created).
 * https://invoicewoo.com/
 * https://nakedcatplugins.com/product/invoicing-with-invoicexpress-for-woocommerce-pro/
 */
add_action(
	// When the order is created via the checkout
	'woocommerce_order_status_on-hold',
	function( $order_id ) {
		$order = wc_get_order( $order_id );
		// If the payment method is "Bank Transfer"
		if ( $order->get_payment_method() === 'bacs' ) {
			// Do something for BACS payment
			$order->add_order_note( 'BACS payment detected - Issue proforma invoice automatically' );
			do_action( 'woocommerce_order_action_hd_wc_ie_plus_generate_proforma', $order, 'automatic' );
		}
	}
);