Request:

{
    "jsonrpc": "2.0",
    "method": "trigger.get",
    "params": {
        "triggerids": "1808353",
        "output": [
            "url"
        ]
    },
    "id": 1
} 

Response:

{
    "jsonrpc": "2.0",
    "result": [
        {
            "triggerid": "1808353",
            "url": "{$CONFLUENCE_URL}"
        }
    ],
    "id": 1
} 

Expected:
In the UI on the Problem page the macros in the trigger url expand well, the API should have the same behaviour, either by default or with an additional option (like expandURL) that for some unknown reason wasn't created/was missed, although similar ones are present for the trigger name, description and expression.