How to link calculated field form to product

Hi there,

I would like to offer a product on my site, based upon the information of the customer. So for example the customer needs to fill in their weight first, before they can order. I need to receive this information with their order. How do I do this?

I have installed calculated field forms, and with this i can make the form, but i’m having difficulty connecting it to a product in shop.

Can somebody offer help?

Thanks a lot.
website is http://www.elyxer.eu