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.
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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
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Don't forget to save your petition!