wc-auto-add-product-to-cart.php

/**
 * Automatically add product to cart on visit.
 */
function wb_add_product_to_cart() {
	if ( ! is_admin() ) {
		$product_id = 38; // replace with your own product id.
		$found      = false;
		// check if product already in cart.
		if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
			foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
				$_product = $values['data'];
				if ( $_product->get_id() == $product_id ) {
					$found = true;
				}
			}
			// if product not found, add it.
			if ( ! $found ) {
				WC()->cart->add_to_cart( $product_id );
			}
		} else {
			// if no products in cart, add it.
			WC()->cart->add_to_cart( $product_id );
		}
	}
}
add_action( 'template_redirect', 'wb_add_product_to_cart' );