Create a new application for an existing candidate. The body accepts one of two shapes: a candidate application (requires candidate_id + job_id) places the candidate at the job's first interview stage (or initial_stage_id when supplied); a prospect application (requires candidate_id + prospect: true) creates a prospect, optionally attached to one or more jobs via job_ids, a prospect pool via prospect_pool_id + prospect_pool_stage_id, or prospective department/office. To create a candidate and their first application in a single call, use POST /v3/candidates with a nested application object.