You can have one today without talking to anyone.
How do I request it?
Request a key at https://www.moonworkers.co.uk/api. The credentials arrive by email: an API key and a client secret, sent together on every request as the X-API-Key and X-Client-Secret headers. See "Authenticating with the Payroll API".
Which URL do I call?
The sandbox has its own base URL: https://payroll-api-sandbox.moonworkers.co.uk. Production is https://payroll-api.moonworkers.co.uk. Every path ends in a trailing slash, for example /api/employer/.
What does the sandbox include?
The full API, with payroll calculations and documents exactly as in production, on test data. Nothing reaches HMRC's live service: RTI submissions run in HMRC test mode. Sandbox use is free, with no commitment and no approval step. Keep in mind that we reset sandbox data periodically, and that rate limits are lower than in production. If you plan to load test, tell us first.
Can I try calls in the browser?
Yes. The Swagger UI on the sandbox, at https://payroll-api-sandbox.moonworkers.co.uk/api/docs/, lets you run any endpoint with your key against the sandbox.
I lost my client secret, or it was exposed
The secret is shown only once, in the credentials email. Contact us and we will revoke the pair and issue a new one. Treat the secret like a password: never commit it to source control or paste it into a ticket or a chat, including this one.
Can I see my sandbox data in a user interface?
Yes. The sandbox front end is at https://payroll-sandbox.moonworkers.co.uk/auth/login. It shows the same data as the API, which helps when you want to check what your integration has created.
Where do I start?
Read the API reference at https://payroll-api.moonworkers.co.uk/api/redoc/, then create an employer, a pay schedule and an employee, and run a first payroll. "Payroll API resource map" lists what each part of the API does. To test HMRC submissions, see "Can we test HMRC submissions and DPS notices in the sandbox?".