Woocommerce Custom Add to Cart URL

I have a question about adding a product to cart via a url. Right now I can use http://example.com/cart/?add-to-cart=49 and it will add the item to the cart. I am using a plugin that adds custom text input for each product. Plugin - Additional WooCommerce product options on codecanyon.

I need to add a parameter to my add to cart url where I can add an item to cart but also pass the value to this user input via the url. Ideally, my url would look like http://example.com/cart/?add-to-cart=49&textbox=myvalue . I'm really not sure how best to approach this. I don't mind using this plugin if there is a better way to make it work. Thanks for your help.