ESignIframeSessionResponse
iframe_url in the partner application iframe and deliver session_key to the iframe with the realaml.esign.session_key postMessage handshake.{
"iframe_session_id": "dcdee395-4335-45c8-ae30-12a2738d2982",
"document_id": "b792e8ae-2cb4-4209-85b9-32be4c2fcdd6",
"iframe_url": "http://example.com",
"session_key": "string",
"expires_at": "2019-08-24T14:15:22.123Z"
}