Maintaining External Reference ID across surveys?

Thanks! I tried that with some test surveys, and I can see the Custom ID field when I download the data for Survey A, but when I download the data file of responses to Survey B, the Custom ID field is blank. I set it in the Survey B Survey Flow as Custom ID (pulled from Contact List) and left it blank so it would be filled by panel (same as for Survey A). Is there something I'm missing? Is it not capturing the IDs from A because people are coming to B via a redirect at the end of Survey A? They're not being contacted a second time. It's just an automatic kick out to the next survey.

@JBaron Did you add them to your contact list at the end of Survey A? You'll need to do that in order to have the info in Survey B populate. If not, you can see how to set up a contact list trigger here.

@JBaron, If you are redirecting from survey A to survey B, then the respondents in survey B aren't going to be associated to a contact list unless you include an authenticator in survey B. Assuming you don't want to add an authenticator, you can pass the External Reference ID from survey A to survey B as a url parameter. You would pipe the value into your redirect url. In survey B, just define the name of the url parameter as an embedded data field (anything other than ExternalReferenceData) and leave it blank to be populated from the url.