Usually you’d do this by adding an input element on the product page which then adds line item properties when the variant is added to the cart (ref https://shopify.dev/docs/themes/architecture/templates/product#line-item-properties).
However, the SparkLayer app bypasses the Shopify product form entirely, and manages it’s own cart. That means you would need to use the SparkLayer SDK to add your custom field at the product level.
Unfortunately, this is going to be pretty involved as there isn’t any kind of built-in support for this. There’s an order level reference field built in to SparkLayer, but that won’t be product-specific.
The good news is that I’ve spoken to SparkLayer about this in the last couple of days, and they have said this feature will be coming in Q1 (estimated), so it might be easier to wait until it’s rolled out by them.
If it’s urgent and you need a developer to come up with a (probably quite complex and expensive) customisation to make this work on SparkLayer before then, happy to discuss that with you.