functions.php

<?php

function auto_send_invoice_after_creation( $invoice_id ) {
	$invoice = SI_Invoice::get_instance( $invoice_id );
	$client = $invoice->get_client();
	$client_users = $client->get_associated_users();
	if ( !empty( $client_users ) ) {
		do_action( 'send_invoice', $invoice, $client_users );
	}
}
add_action( 'si_recurring_invoice_created', 'auto_send_invoice_after_creation' );