First create your ShareProgress page, then paste your share page URL as your "Redirect URL in Salsa.
After you create a page on ShareProgress, you will receive a url for your newly-created share page. Highlight this url and copy it on to your clipboard.
With that URL copied, log into your petition on Salsa. Click on the tab labeled Follow-Up. Select Redirect to Another Page in the dropdown menu and paste your share page url in the field labeled Redirect Path
Don't forget to save your petition!