You are currently unable to add a custom text box per product. You will be able to add a text box where your customers can add in their custom requirements at your checkout. Go into Settings > Cart/Payment > Check out – Custom Field and tick the 'Enabled' box.
You can add in the title ‘Special Requests’ or whatever you wish and we would recommend choosing the 'Text Field' option in the Custom Input Type section. This text box will be displayed at the payment section of check out.