I am trying to pass some data fields to a 3rd party application

I have included the following information in the URL for the Thank You link: https://app.webinarjam.net/auto-register?webicode=e76ae27951&memberid=32412&firstname={firstName}&email={email4}&countrycode=US&schedule=1

The bracketed information is passing, rather than the field values.

Could you please check if your form has enabled the send post data option? 

Here is also a guide that will help you with detailed steps about it: https://www.jotform.com/help/213-Send-Submission-Data-via-Post-Method-and-Thank-You-Settings 

You could also provide us the form you're working on so we can take a look on it.