Feature Request: Use Survey Response as distinct_id for Hosted Surveys

Feature request

Is your feature request related to a problem?

Currently, hosted surveys require the distinct_id to be passed as a URL query parameter before the survey loads. This means users must already be identified before they access the survey.

For use cases where we want to collect identifying information (like an email address) within the survey itself and use that to identify the respondent, there's no way to do this. The survey response is captured, but it remains anonymous unless the distinct_id was pre-populated in the URL.

Describe the solution you'd like

I'd like the ability to designate a specific survey question (like an email or user ID field) as the distinct_id for that response. When a user submits the survey, PostHog would use the value from that designated question to identify the user, similar to how posthog.identify() works in the SDK

This would allow hosted surveys to:

• Collect responses from anonymous users and identify them based on their answers
• Eliminate the need to pre-populate the distinct_id in the URL for every recipient
• Enable better tracking of survey respondents who weren't previously identified in PostHog

This would be particularly useful for surveys sent via email or shared publicly where we don't have the user's distinct_id upfront, but want to identify them based on information they provide in the survey.

Describe alternatives you've considered

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/52956 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/55575)

Debug info

Kind: support

Target area: surveys

Report event: http://go/ticketByUUID/d6a09d1a-7124-48cd-b385-1019da0d3ff7

Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/019cd84c-094a-7fbd-9699-60b6223f1fe5?t=1935

Exceptions: https://us.posthog.com/project/2/error_tracking?filterGroup=%7B%22type%22%3A%22AND%22%2C%22values%22%3A%5B%7B%22type%22%3A%22AND%22%2C%22values%22%3A%5B%7B%22key%22%3A%22%24session_id%22%2C%22value%22%3A%5B%22019cd84c-094a-7fbd-9699-60b6223f1fe5%22%5D%2C%22operator%22%3A%22exact%22%2C%22type%22%3A%22event%22%7D%5D%7D%5D%7D

Location: https://us.posthog.com/project/93841/ai?chat=0f5232a5-660a-4207-be6f-7b889b93f35a

Persons-on-events mode for project: person_id_override_properties_on_events

Start by tracing hosted survey loading, URL distinct_id handling, response capture, and submission behavior. Define how a designated question's value becomes the respondent's distinct_id for anonymous submissions, then verify the behavior against the linked support scenario and the existing posthog.identify semantics.