Ok, so I think that it needs to look like this:
carrot.postAction("ACTION\_ID", nil, nil, { object\_type = "OBJECT\_TYPE", title = "New Object Title", description = "New Object Description", template = "TEMPLATE\_ID", image\_url = "http://placekitten.com/512/512"})
And that new parameter ‘template’ needs to be the identifier of the instance that you said “Use as Template”.
Then in the web-tool, put “{{title}}” for the title, “{{description}}” for the description etc.
You can also add more data and such, so for example the description in the web tool could be: “I just reached level {{level}}!” and you would pass along level = “5” in the call, if that makes sense.