PandaDoc's API at https://api.pandadoc.com/public/v1 creates documents from templates and sends them for signature.
Generate a key under Settings → Integrations → API (sandbox keys available). Send it as Authorization: API-Key <key>. In the examples it's written {{PANDADOC_API_KEY}}.
Full reference: https://developers.pandadoc.com/reference/about
curl "https://api.pandadoc.com/public/v1/documents?count=10" \
-H "Authorization: API-Key {{PANDADOC_API_KEY}}"
curl https://api.pandadoc.com/public/v1/templates \
-H "Authorization: API-Key {{PANDADOC_API_KEY}}"