A few years ago I saw a demo where a customer merged the Lead ID into to append to a URL in an email template. The URL pointed the email recipient to a web to lead form which had the Lead ID pre-populated from the merged Lead ID field contained in the URL. By doing this, the customer was able to send out event registrations to existing Leads and have them register for the event (Campaign) via a web to lead form without needing to provide information which they had previously provided.

I can setup the email template to include the merge field, however can't work out how to get the lead ID field on my web to lead form to populate. 

I thought the URL should look like this, although this just loads the web to lead form without inserting the Lead ID - http://www.<mywebsite>/html?Lead.Id=<lead record id>

Any suggestions?