I can see how it could be useful to have some indication on Zendesk that a topic has been marked as solved on Discourse. Ideally, what would you want to happen when a topic is marked as solved on Discourse? Should the Zendesk ticked be closed? An alternative to that might be to add a tag to the Zendesk ticket when a topic is marked as solved on Discourse.
A TopicCustomField is created with it’s name set to "accepted_answer_post_id".
A Solved Event webhook can be configured to send a POST request with details about the solution. I’m not sure if Zendesk can be configured to handle incoming POST requests. Do you know if that’s possible?