Pass parameters from Website to Dynamics Portal

Hi 

We have a list of products on our website and we would like a user on our website to click on a product, redirect to the Dynamics Portal and automatically populate the Product lookup on a custom form available on a Dynamics Portal for customer self service. 

We are aware that in order to populate the lookup we require the GUID, Entity Name and the Entity ID for the record and pass these parameters from the website to the form.

I am looking for a bit more details on how to achieve this. Do we need to send the values with XML / JavaScript?

Would someone please advise on the easiest way to achieve this

Thanks
Tony