Welcome to the Losant Forums! Just a few things to get us debugging. We see 403 errors when either no authorization token is provided, or the authorization token does not have the proper scope. To debug this, I would recommend checking that your permissions for the Application API Token are correctly scoped. I would also recommend checking out this forum post which could provide some helpful information.
One additional way to debug is through Postman. Postman is a tool that can be used for POST requests, and will format the request for you, you just have to fill out the parameters. Here is an example of Postman:
Let me know what comes from this debugging!
Thanks,
Julia