Not entirely sure what the precise situation is, but it's against the HubSpot ToS to store any private/sensitive information in HubSpot (e.g. SSN, credit card info, info protected by HIPAA, etc.). Just wanted to throw this out there since the subscription validation service you mentioned was a "healthcare professionals" service.
Now that that's out of the way; I'd start by saying that the method you choose depends more on how the 3rd party service accepts the data involved. For Example:
- You could create a custom HTML form, submit that data to your server, then pass it along to the 3rd party service and do whatever other processing needs to occur before passing the info into HubSpot via the Forms API.
- Or, you could submit the form into HubSpot via a normal HubSpot form, but set up a workflow with a webhook action that sends the resulting contact data to your server, where you can push any necessary details to the 3rd party service.
Happy to get more specific if you have any more details on what your implementation would need to look like!