An API key can be generated for each of your forms from the "API Keys" tab. You should keep your API key private and not use it client-side unless you want your data to be publicly available. You can regenerate or delete the API key at any time.
With your API key, you can now make authenticated HTTP requests by setting the Authorization
header:
$ curl -H "Authorization: Bearer API_KEY" https://postform.com/api/v2/submissions