functions.php

/**
 * Change the subscription thank you message after purchase
 *
 * @param  int $order_id
 * @return string
 */
function custom_subscription_thank_you( $order_id ){

    if( WC_Subscriptions_Order::order_contains_subscription( $order_id ) ) {
        $thank_you_message = sprintf( __( '%sThank you for purchasign our subscription! Visit %syour account%s page to know its status.%s', 'woocommerce-subscriptions' ), '<p>', '<a href="' . get_permalink( wc_get_page_id( 'myaccount' ) ) . '">', '</a>','</p>' );

        return $thank_you_message;
    }

}
add_filter( 'woocommerce_subscriptions_thank_you_message', 'custom_subscription_thank_you');