I’ll try 
I grabbed the link of the topic where the small action post is, and popped /change-owner.json on the end, and added it to the top bar as a POST request (and entered my API-Key API-Username, plus Content-Type into the Headers tab):
I then looked up the post_id of the small action post I wanted to change in the post.json (eg. /t/tag-test-topic-one/762/2.json) and added the post_ids[] and username Keys and Values to the body tab, under ‘form-data’:
Then clicked the ‘Send’ button. 
