It appears to need a key which can be obtained from a request form – instructions are in the Github, but I can’t post a link to the request form since I’m a new user and can only have 2 links in my post.
That repo has been archived. Unfortunately, their website states that even though the API does exist, access to it isn’t free:
To be able to access APIs, customers need the following credentials:
For all Weather APIs other than the Nautical API:
Client ID.
Client secret.
The API credentials are provided by DTN Customer Service once a trial/contract has been agreed. Once the API credentials are created, the trial customer receives a welcome message including the relevant credentials.