I'm struggling with this code

Hi,

I’m adding this to the Product Short Description to try and change the “Add to Cart” to “Pre-order”, but it doesn’t work (N.B. the PHP works fine when adding it to functions.php, but that changes all the buttons and I only want it for some pages):

[insert_php]

add_filter( 'add_to_cart_text', 'woo_custom_cart_button_text' );

add_filter( 'woocommerce_product_single_add_to_cart_text', 'woo_custom_cart_button_text' );

add_filter( 'woocommerce_product_add_to_cart_text', 'woo_custom_cart_button_text' );
function woo_custom_cart_button_text()
{
return __( 'Pre-order', 'woocommerce' );
}

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[/insert_php]

Any idea why this is?
The theme I’m using is woocommerce one called Blaszok.

Any help will be deeply appreciated as I’m at my wits end with this ??