Once you arrive to the Domain Mapping System settings, check the box at the bottom labeled "Support for query string parameters (e.g. - UTM, etc)":
Query string parameters have a wide variety of use-cases and formats, such as this example:
You might use query string parameters for any of the following use-cases and many others:
Google UTM parameters
Prefilling form field data on a page
Query string parameters should not be confused with subdirectories or pages of your site, like this:
Once you've enabled support for query string parameters, you should see the data "pass-through" your mapped domain, so data will be tracked in your corresponding platform for the primary site domain.
In the future, we hope to add support for query string parameters to mapped domains independently of the primary domain. If you need this feature, please submit a ticket.