curl --request POST \
  --url https://api.example.com/brokerage-firms/{brokerageFirmId}/signature \
  --header 'Content-Type: application/json' \
  --data '
{
  "deliveryMode": "EMAIL"
}
'

{
  "id": "<string>",
  "signers": [
    {
      "customerId": "<string>",
      "url": "<string>"
    }
  ]
}