Iris here from Shopify Support. Just in case you were not aware and do speak Dutch, this is our Dutch forum and any question can be writen in Dutch. For questions in English it is properly easier to consult our English Forum, as this is read and answered by a much larger group than here in the Dutch forum. By the way this is a great question and I had to dig deep for a good solution, but I think I have found one.
Just some more information to explain my though process. Normally you can collect customization information for products using line item properties. Line item properties are custom form fields that you can add to the product page, allowing customers to make choices or add information about a product. For example, if you offer product engraving, then you can use line item properties to let customers enter the text that they want engraved on the product. But in this case, it will do this to both variants so I don't think this is a great solution.
Order notes and cart attributes might be a solution as Order notes, which are available in every free Shopify theme, let you capture special instructions on how to prepare and deliver an order. You could leave instruction in the variant description that the customisation information is needed at check out. Cart attributes are specified by customers on the cart page, and are used to record additional information about an entire order, so perhaps also an option.
An alternative is that you can install a free or paid product customization app from the Shopify App Store to easily add custom fields to your product page. You could consult with the app designers to see if there are any possibilities to limit this to one variant.
I have found that properly the best option would be to use the app Bold Product Options as you can see here it is possible to select if there is customisation needed or not.
Hope this helps.