Add Product to WooCommerce Cart Automatically

add_action( 'template_redirect', 'snippet_add_product_to_cart' );

function snippet_add_product_to_cart() {

// select ID
$product_id = 21874;

//check if product already in cart
if ( WC()->cart->get_cart_contents_count() == 0 ) {

// if no products in cart, add it
WC()->cart->add_to_cart( $product_id );

}

}