Second Party Prefill API

The Second-Party Prefill API enables Engine partners to securely pre-populate customer application data (including PII such as Name, Address, SSN, and Income) into Engine's embedded application consume experience.

You send the consumer's data server-to-server, receive a short-lived prefillToken plus the markup needed to render the experience, and then load that experience for the consumer. The personally identifiable information (PII) never travels through the browser.

If you want to pass full data prefill with consents programmatically, use Engine's Offer Wall API instead — it provides a more seamless end-to-end experience for that use case.

The Prefill API is built primarily for partial prefill into Engine's hosted Partner Pages and embedded experiences.

Key Benefits

This integration reduces consumer friction, eliminates data entry fatigue, and significantly improves conversion rates.

• Better UX: consumers start the application with their information already filled in.

• Security: PII is exchanged server-side, so it is never exposed in browser URLs, query parameters, or static HTML.

  • The PII you send is encrypted at rest; the prefillToken is an opaque, short-lived reference that Engine exchanges for that data when the experience renders.

  • There are limitations on the number of times a token can be used (maxUses is 3) and how long it's valid for (expiresInSeconds is 180)

• Flexible: supports partial prefill; any field you omit is simply collected from the consumer in-flow.

Works Across Product Marketplaces

This guide is split by product category because the accepted request fields and the embed markup differ between them. Start with the page that matches your product:

• Personal Loans, SLR, & ALR for Engine's lending experiences (including Personal Loans, Student Loan Refinance, Auto Loan Refinance).

• Credit Cards for Engine's credit card marketplace.

The rest of this page covers the parts that are the same regardless of product: how the flow works, authentication, the token lifecycle, your frontend options, and best practices.

How it Works

1

Submit User Data

Your server sends the consumer's lead data (PII, financial info, etc.) to the Prefill API.

2

Receive a prefillToken

Engine validates the request and returns a short-lived prefillToken plus a destination partnerPageUrl and an embedSnippet.

3

Initiate the Frontend User Experience

You either load the embed snippet on your own site or redirect the consumer to the returned partner page URL. Both are returned in every response — you choose which to render.

4

Consumer Completes Form

The consumer lands on the application with their data prefilled and enters any required fields you didn't send.

5

Consumer Reviews and Submits Form

The consumer reviews the data, provides consent, and submits to receive prequalified offers.