WooCommerce Automatically add product to cart on site visit

add-product-to-cart.php

<?php
/*
 * This code goes into theme functions.php or a custom plugin
 */
 
 /**
  * Add product to cart on page load
  */
function add_product_to_cart() {

	if ( ! is_admin() ) {
		$product_id = 123; // Product ID to auto-add
		$variation_id = 124; // Set to 0 if no variation
		
		if ( empty( $product_id ) ) {
			return;
		}

		// Get WC Cart
		$cart = WC()->cart;
		
		// Get WC Cart items
		$cart_items = $cart->get_cart();

		// Check if product is already in cart
		if ( 0 < count( $cart_items ) ) {
			foreach ( $cart_items as $cart_item_key => $values ) {
				$_product = $values['data'];

				// Product is already in cart, bail
				if ( $_product->get_id() == $product_id ) {
					return;
				}
			}
		}

		// Add product to cart
		$cart->add_to_cart( $product_id, 1, $variation_id );

		// Calculate totals
		$cart->calculate_totals();
		
		// Save cart to session
		$cart->set_session();
		
		// Maybe set cart cookies
		$cart->maybe_set_cart_cookies();
	}

}
add_action( 'template_redirect', 'add_product_to_cart' );