I have implemented Adaptive Payment in my website. First It was Standard Payment so in that i was passing a custom parameter in the form and i was getting it i
I agree with @jimy to use the querystring, but in PayPal Adaptive the parameter is not notify_url.
For PayPal Adaptive use the ipnNotificationUrl field of the Pay or Preapproval request
Source: Identifying Your IPN Listener to PayPal
[Edit]
As per @jackvsworld suggests in PayPal Adaptive use parameter as ipnNotificationUrl.
I did this by passing into GET parameters of notify_url
For e.g. http://www.mywebsite.com/index.php?param_1=value¶m_2=value