Parameter value in Data Factory Triggered pipeline
Finally I was able to fix this by creating the trigger using a JSON code as shown below:

{
    "name": "yourTriggerName",
    "properties": {
        "runtimeState": "Started",
        "pipelines": [
            {
                "pipelineReference": {
                    "referenceName": "YourPipelineName",
                    "type": "PipelineReference"
                },
                "parameters": {
                    "windowStart": "@trigger().scheduledTime"
                }
            }
        ],
        "type": "ScheduleTrigger",
        "typeProperties": {
            "recurrence": {
                "frequency": "Day",
                "interval": 1,
                "startTime": "2018-07-11T17:00:00Z",
                "timeZone": "UTC",
                "schedule": {
                    "minutes": [
                        20
                    ],
                    "hours": [
                        19
                    ]
                }
            }
        }
    }
}


And making emphasis of course, in line below:

"parameters": {
                        "windowStart": "@trigger().scheduledTime"


After that, the copy activity started to work as expected.