Usually you you would use "auto_capture": true, and it is not necessary to make this call.

When you use invoice payment, you must use another call to capture the Payment Transaction.

In order to start the authorized Smart Transaction, you only need to call POST Smart/Transactions/STX_xxx/start.

Request
POST /api/v2/Smart/Transactions/STX_33PXAW2YN2NJTPM5KPGMK7QF5PBVA2/start HTTP/1.1
Host: connect-testing.secuconnect.com
Authorization: Bearer qb56tjj1bcvo9n2nj4u38k84lo
Accept: application/json

If everything is fine, the API responds with 200 OK:

Response
HTTP/1.1 200 OK
Content-Type: application/json
{
"object": "smart.transactions",
"id": "STX_33PXAW2YN2NJTPM5KPGMK7QF5PBVA2",
// ...
"transactions": [
{
"object": "payment.transactions",
"id": "PCI_FDBREW9ZS7P6FTN4ZY2ATC6NK0QWO7"
}
],
"created": "2020-03-27T10:55:23+01:00",
"updated": "2020-03-27T10:56:41+01:00",
"status": "ok",
// ...
}

Status ok, received or collection means you can deliver. Status pending means you have to wait for one of these statusses. Status failed means the payment failed, and you must repeat the payment process.