How do I add Salesforce field data to a widget form?

I have an Echosign widget posted to a webpage. I'd like to create a custom button in Salesforce.com that pre-fills one of the fields in this form. Here's what the URL looks like:

"http://www.efolder.net/ccform?" (I added the quote marks)

This url is used in the button called Credit Card Form Link shown below.

I'd like to pre-fill one of the fields in this form with the text from a Salesforce.com field called {!Opportunity.OrderNumber__c}. There is a corresponding field in the form (the first field).