curl --request POST \
  --url https://developer-api.squads.so/api/v0/smart_account/transaction/signers/add \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "smart_account_address": "<string>",
  "transaction_signers": [
    "<string>"
  ],
  "smart_account_signers": [
    {
      "address": "<string>",
      "permissions": [
        "CAN_INITIATE"
      ]
    }
  ],
  "fee_config": {
    "currency": "sol",
    "payer_address": "<string>"
  }
}'

{
  "transaction": "<string>",
  "fee": {
    "amount": "<string>",
    "amount_decimal": "<string>",
    "currency": "<string>",
    "sol_equivalent": {
      "amount": "<string>",
      "amount_decimal": "<string>"
    }
  }
}