How can I set the topics on a post at submit time using the API?

You should pass an array of topics like this:

{

"content": "Test Content",

"type": "post",

"files": [],

"images": [],

"group": "main-feed",

"topics": [{

 "name": "Baseball",

 "_id": "5c4800743d9228aa7c238598"

}]

}