In order to make API requests, you have to request an app review. Dev mode is allowed for some time/number of requests, but when your integration is in production, you should request an app review and switch the app to Live mode. An app in Dev mode could stop working as expected at any moment.
One of the way you can receive leads from Marketplace Vehicles is Messenger. Depending on the communication channel you have chosen, you or your dealerships will receive leads in their Facebook professional pages. If you have a chat platform that handles leads for autos, you can integrate with Messenger to allow potential buyers to communicate with dealerships.
Please see the Messenger Platform docs for more detail.
Dealership pages will need to have Messenger activated in order to receive leads.
If you are looking to work with an existing chat platform, you can find the list of them here.