How can I prefill form with Confidential Information?

If I pass the parameters in the URL to prefill the form, the content is transmitted in plain-text, even though the form is an HTTPS form.

Is there a way for me to pass data into a form to pre-populate the form while still keeping the data secure in transit?

The URL parameters would indeed be in plain text if you use the pre-population feature. Unfortunately there is no other way to prefill the form aside from the URL parameters. The only way to ensure that the confidential information remains secure is if you let the user fill in their confidential information instead of pre-populating it via URL parameters. 

Otherwise, you can choose to send the URL for the pre-populated form to your user via a trusted email service.
Regards.