IMPORTANT NOTICE: Effective December 20th, 2018, the HubSpot Public Contact URL will be no longer accessible to non-HubSpot users. The official announcement from HubSpot about this change can be found here.
HubSpot has a Public Contact URL that they make available to sync via their Contacts API and it's an available field that you can map within your Formstack Sync account. This is a valuable field that is often sent over to a CRM so that a sales or account management team can easily click through and view a particular contacts full profile as it is within HubSpot. There's a valuable timeline view that shows the history of a contact with regards to online marketing activities that have taken place.
When mapping this field in Sync, you'll want to place it at the bottom of the other fields you are mapping to in the other system(s). You will also want to set the HubSpot field to Read-Only. Here's a screen shot of how this should look.