I used to pull this information by adding the following to the query string:
customColumns=%23%23EXTERNALTICKETID%23%23
which would give you an external "customColumns" object in the returned payload with the fields you request, then you could parse ##EXTENRALTICKETID## by splitting by the colon char and strip the leading space.
"customColumns": {
"##EXTERNALTICKETID##": "SCC ServiceNow Integration : INC5132740"
}
I've just had a look to see if you could do the same with alertExternalTicketURL but it seems to be in the default output, so maybe the change you asked for is already in place? (bits removed for brevity!)
{
"id": "DS8824603",
"ruleId": 30,
"alertExternalTicketUrl": {
"servicenowIncidentLinks": {
"INC5132740": "https://xxxxxxx.service-now.com/now/nav/ui/classic/params/target/incident.do%3Fsys_id%3Db05f563a1b962a10438ced79b04bcb81"
}
},
"tenant": "undefined",
"alertValue": "1.0",
"sdted": false,
"SDT": null,
}