Why is it Required?
We have an option in the backend for the Optin step to enable this functionality that will pass the fields values to the next page from the URL parameters.
By passing the variables from the URL you can create a custom code to display the personalized message or use it for certain scenarios wherein you can send the data to your CRM System using the custom code.
How to use this feature?
Visit and edit the Optin Flow, go to the Optin Settings > Settings > Enable Pass Fields as URL Parameters
On enabling this setting, you will see the first_name added by default, you can pass the value as per your requirement and see it mapped to the URL and can be used as per your requirement.