How do I compose and send for signature in one call?

Use /api/v1/flow instead of /api/v1/compose/documents/generate whenever the document is going out for signature the moment it exists — one request, one round trip, and no window where you are holding a document that has not been sent yet:

curl -X POST https://your-host/api/v1/flow \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: order-4471" \
  -d '{
    "data": { "customer": { "name": "Acme Srl" } },
    "settings": { "templateId": "contract-standard-8bc99b" },
    "signature": {
      "subject": "Service agreement",
      "signers": {
        "A": { "email": "anna@acme.example", "fullName": "Anna Rossi",
               "fields": [{ "kind": "signature", "page": 1,
                            "x": 0.1, "y": 0.78, "width": 0.32, "height": 0.09 }] }
      }
    }
  }'

Where the signature goes. The usual way is a tag in the template, {{ signature.A }}, written like any other field: signer A then needs no fields and signs where the tag is. The alternative is fields, as here: x and y are fractions of the page from the top-left corner (0.1 = a tenth of the way across). How to read them off a PDF, and everything else about placing a signature: Where the signature goes on the page.

Always send the Idempotency-Key — this call charges twice and emails a real person, and a retried request without one does both again. Full field list and what comes back: Compose and sign, in one call.