Initiates an electronic signature workflow for one or more documents and signers. Requires X-API-KEY and X-Client-ID headers. signer_position.appearance accepts either a preset position string (top-left/top-right/bottom-left/bottom-right) or an array of {x1,y1,x2,y2} coordinate boxes; the page(s) to sign are chosen via page_number. Malformed coordinates are rejected with HTTP 422; a structurally valid request the signing platform cannot place (e.g. multiple boxes when not configured) returns HTTP 200 with status "failed" and a vendor error_code such as es_102.