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.
You will be given your share page URL after you create a share page.

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
Paste your ShareProgress into Salsa's Follow-Up tab to set it as your after action page.

Don't forget to save your petition!