Add comment to task

I wish to add a comment to a task with the API

It seems I must use POST
https://app.liquidplanner.com/api/v1/workspaces/workspace_id/treeitems/treeitem_id/comments ?

I have tried using it and it seems to work sometimes and others not. I add the tree item id as my task

comment.item_id ??? What is this? I have tried using 0 and it worked a couple of times and added 2 comments. I tried other values and sometimes it succeeds, but when I look at the comments it isn't there. Many times I just get a 500 error even though I am doing something which worked previously. Any help with this please?