The Custom Endpoint integration currently is sending JSON as the request, but we could consider adding additional options there. I've opened a Feature Request if you'd like to add any more details there:
In general, the Custom Endpoint is best used when you control the other end of the request and control how you are receiving the data there. While it's possible that the Custom Endpoint can work with specific platforms, there may be several other considerations that will need to be addressed as you work toward a successful request such as authentication and the format of the fields being sent in the request - and in several cases, it may not be possible at all with the default settings or may require some additional configuration updates to get those things working.
For integrations with a specific platform that may require specific settings, it's Sprout Forms does have an Integrations API where you can add your own custom Integration for an editor to select and use.
In your case, that could mean that you create a Pardot Integration, configure your Pardot API credentials within your Craft and Integration module settings, and have the integration call out to the Pardot API to present the user fields or options that are retrieved from your Pardot account directly.