Raise a customer request in Jira Service Management via the servicedesk REST API

Documented steps

1. Use the JSM base path /rest/servicedeskapi/ (not /rest/api/3/). Authentication is the same as the Jira platform API.
2. Find the service desk: GET /rest/servicedeskapi/servicedesk?start=0&limit=50. Each entry has id, projectId, projectKey, projectName. If you already know the ID, GET /rest/servicedeskapi/servicedesk/{serviceDeskId} instead — the list call is slow on sites with many service desks.
3. List request types: GET /rest/servicedeskapi/servicedesk/{serviceDeskId}/requesttype?start=0&limit=50. Note the requestTypeId you need. The serviceDeskId path segment also accepts a project key.
4. Discover the fields a request type expects before submitting, so requestFieldValues matches the portal form.
5. Raise the request: POST /rest/servicedeskapi/request with body {"serviceDeskId":"10","requestTypeId":"25","requestFieldValues":{"summary":"Laptop will not boot","description":"Fails at POST since this morning."}}.
6. To file on behalf of a customer add "raiseOnBehalfOf":"<accountId or email>"; to add watchers add "requestParticipants":["<accountId>"].
7. If you are sending ADF-formatted values rather than plain text, set "isAdfRequest": true.
8. Read the 201 response for issueId, issueKey, currentStatus and _links.web (the portal URL to give the customer).
9. Only add the header 'X-ExperimentalApi: opt-in' for endpoints the docs explicitly flag as experimental (for example attachTemporaryFile). The servicedesk, requesttype and request endpoints above do not require it.