Off the top of my head I think Postmark requires an array that can have all the Template fields you set in your Postmark Template - so you can send a bulk email with custom info in it for every Contact (unique name, User links, etc)

So more like:

{“templatemodelid”: “123”, “FromName”:“Keith’s Email”, “name”: “Joe Smith”, “email”: “joe@example.com”, “messagebody”:“Hello Joe, this is the email message”},
{“templatemodelid”: “123”, “FromName”:“Keith’s Email”, “name”: “Frank Doe”, “email”: “frank@example.com”, “messagebody”:“Hello Frank, this is the email message”}