Its the ID of the queue item.
For getting through API call
Use GET > Odata\QueueItems endpoint.
Key in the result is what you need
{
"QueueDefinitionId": 3899,
"OutputData": null,
"AnalyticsData": null,
"Status": "Successful",
"ReviewStatus": "None",
"ReviewerUserId": null,
"Key": "6ea50a9c-b657-4df3-bfe6-d1893c4f31ec",
"Reference": "82086",
"ProcessingExceptionType": null,
"DueDate": null,
"RiskSlaDate": "2022-02-17T13:27:43.895Z",
"Priority": "High",
"DeferDate": null,
"StartProcessing": null,
"EndProcessing": null,
"SecondsInPreviousAttempts": 0,
"AncestorId": null,
"RetryNumber": 0,
"SpecificData": "{\"DynamicProperties\":{\"EmployeeName\":\"John Kensington\",\"ContactNumber\":\"+4457899\",\"RelocationDistance\":500,\"Description\":\"Home relocation\",\"ExpenseCode\":\"UI234\",\"Amount\":1000,\"RequirePass\":true}}",
"CreationTime": "2021-03-19T17:37:35.893Z",
"Progress": null,
"RowVersion": "AAAAAABQET4=",
"OrganizationUnitId": 39419,
"OrganizationUnitFullyQualifiedName": "Shared",
"Id": 2499339,
"ProcessingException": null,
"SpecificContent": {
"EmployeeName": "John Kensington",
"ContactNumber": "+4457899",
"RelocationDistance": 500,
"Description": "Home relocation",
"ExpenseCode": "UI234",
"Amount": 1000,
"RequirePass": true
},
"Output": null,
"Analytics": null
}
]
}
To pass manually
you can find it when you open the transaction/queue item details in queue