Magento - 在Marketplace的product_addtocart_form上添加自定义逻辑

I'm creating a Marketplace logic for my webSite but I'm facing a problem with product_addtocart_form form.

I've customized the product page and I'm trying to set a customAttribute and retrieve it on the cart.

Example: The user has 3 options to select on the product page. I want to store the information of which option the user picked and retrieve it on the cart.

Mage::getSingleton('checkout/session')->setData('marketplace_info', $customInfo);

The logic above works and I'm able to get it on the cart. The problem now is that I don't know how to set this information only after the user choose/submit the form.

The form that I'm trying to customize is placed on the view.phtml and the HTML is:

<form action="*magento-url*" method="post" id="product_addtocart_form">

Logic that I want to create:

User select one option (button) > My custom logic set a customAttribute (logic mentioned above) > Magento default logic add product to the cart.

Any idea? Thx

You are doing it wrong way.Add a input tag inside your form. Add your value under the value part of input tag. Like below

<input type="hidden" name="newVal" value="<?php echo 'yourValue'?>">

Now in your cart controller you will easily get this value. And there you can set the value in your session or where ever you want. Dont forget to add a check "if(isset('yourvalue')){}" like this in your controller.