The endpoint to create a Smart Transaction is POST /api/v2/Smart/Transactions:

Request
POST /api/v2/Smart/Transactions HTTP/1.1
Host: connect-testing.secuconnect.com
Authenticate: Bearer qb56tjj1bcvo9n2nj4u38k84lo
Content-Type: application/json
Accept: application/json
 
{
"is_demo": true,
"basket_info": {
"sum": 0,
"currency": "EUR"
}
}

If everything is fine, the API responds with 200 OK and the representation of the Smart Transaction:

Response
HTTP/1.1 200 OK
Content-Type: application/json
...
 
{
"object": "smart.transactions",
"id": "STX_KJXJDUN542NR3WMRHRVU5WWM6ZQEA2",
"merchant": {
"object": "general.merchants",
"id": "MRC_35SZ3R5GGQQP4T0U2T5GFAN9P6C4PG",
"companyname": "John's Bakery"
},
"transactions": [],
"created": "2020-09-22T13:53:44+02:00",
"status": "created",
// ...
"basket": {
"products": [],
// ...
},
"basket_info": {
"sum": 0,
"currency": "EUR",
// ...
},
"is_demo": true,
// ...
}

When you authenticate as device, the merchant is resolved automatically.