Hi Riz -
ONTRAPORT will have a built in Upsell Form process for ONTRApages very soon, and that process will help automate things. We currently do it using our Legacy Landing Pages and special Upsell Forms as described in this article: http://support.ontraport.com/entries/26966104-Creating-an-Upsell-Form
Keep an eye on that article; within the next few days we will be adding the function to ONTRApages and ONTRAforms.
To answer your specific question, you can configure the Thank You page by editing the Smart Form's Settings. In an ONTRApage, select the Smart Form block in the palette on the left, then click the "Form Settings" icon to open the settings page.
