Adding gift message (custom field) to grouped product

How can I allow a gift message to be added on a grouped product?

I have grouped products set up and the child products within, different subscription based products that are not visible individually.

I tried the Flexible Product Add-Ons extension but this only shows the custom fields if the product has a price but a grouped product does not have a price.

I also tried the WooCommerce Product Add-Ons extension but this too, does not allow the ‘Add-on fields’ to be used on grouped products.

I’m just seeking a way to include a textarea for a gift message on the actual grouped product page.

Can anyone suggest a way of doing this?

Thanks in advance.