I have not found relevant API in https://docs.moodle.org/dev/Main_Page
When you say "send a forum post", do you mean from an external system?
You can use the mod_forum_add_discussion and/or mod_forum_add_discussion_post functions to achieve this via web services:
https://docs.moodle.org/dev/Web_service_API_functions
Yes, that's it. Thank you!