duplicating behaviour from a classic asp page

Many years ago I built a website for a customer in classic asp that uses RomanCart as the payment gateway.  This has worked very well, but ive now been given the task of rebuilding the site under .net 3.5


the old site used a submit button that set the action to the romancart url and setup some hidden field values.  I want to duplicate this behaviour in the .aspx page from the onclick event of a button.  Heres the origianl code from the classic asp page.  Can anyone give some some pointers in converting this to c# ?


the submit button



<input type="button" value="Make Purchase" name="btnSubmit"   &nb ...