Add a new headcount slot to an existing job. job_id is required; opening_id is an optional partner-supplied external identifier (e.g. an HRIS or ATS position id) and custom_fields set values on the organization's opening custom fields. Greenhouse enforces an internal limit of 100 open openings per job — requests that would exceed it are rejected. To create a batch of openings alongside a new job, use POST /v3/jobs with number_of_openings instead.