How to download Image of amcharts using export functionality from external button

I have a column chart I want to download the chart using amcharts export functionality but the difference is I don't want to use the amcharts icon instead I want to use my own buttons

/**
 * ---------------------------------------
 * This demo was created using amCharts 4.
 * 
 * For more information visit:
 * https://www.amcharts.com/
 * 
 * Documentation is available at:
 * https://www.amcharts.com/docs/v4/
 * ---------------------------------------
 */

// Themes begin
am4core.useTheme(am4themes_animated);
// Themes end

// Create chart instance
var chart = am4core.create("chartdiv", am4charts.XYChart);

// Add data
chart.data = [{
  "country": "USA",
  "visits": 2025
}, {
  "country": "China",
  "visits": 1882
}, {
  "country": "Japan",
  "visits": 1809
}, {
  "country": "Germany",
  "visits": 1322
}, {
  "country": "UK",
  "visits": 1122
}, {
  "country": "France",
  "visits": 1114
}, {
  "country": "India",
  "visits": 984
}, {
  "country": "Spain",
  "visits": 711
}, {
  "country": "Netherlands",
  "visits": 665
}, {
  "country": "Russia",
  "visits": 580
}, {
  "country": "South Korea",
  "visits": 443
}, {
  "country": "Canada",
  "visits": 441
}, {
  "country": "Brazil",
  "visits": 395
}];

// Create axes

var categoryAxis = chart.xAxes.push(new am4charts.CategoryAxis());
categoryAxis.dataFields.category = "country";
categoryAxis.renderer.grid.template.location = 0;
categoryAxis.renderer.minGridDistance = 30;

categoryAxis.renderer.labels.template.adapter.add("dy", function(dy, target) {
  if (target.dataItem && target.dataItem.index & 2 == 2) {
    return dy + 25;
  }
  return dy;
});

var valueAxis = chart.yAxes.push(new am4charts.ValueAxis());

// Create series
var series = chart.series.push(new am4charts.ColumnSeries());
series.dataFields.valueY = "visits";
series.dataFields.categoryX = "country";
series.name = "Visits";
series.columns.template.tooltipText = "{categoryX}: [bold]{valueY}[/]";
series.columns.template.fillOpacity = .8;

var columnTemplate = series.columns.template;
columnTemplate.strokeWidth = 2;
columnTemplate.strokeOpacity = 1;

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}

#chartdiv {
  width: 100%;
  height: 500px;
}

<script src="https://cdn.amcharts.com/lib/4/core.js"></script>
<script src="https://cdn.amcharts.com/lib/4/charts.js"></script>
<script src="https://cdn.amcharts.com/lib/4/themes/animated.js"></script>
<button id = "image"> export image </button>
<button id = "pdf"> export pdf </button>
<div id="chartdiv"></div>

Now I want to use my own button to export chart as image or pdf using default amcharts functionality , is it possible ?

No problem. You can use the export API.

In your example, just add event listeners and call chart.exporting.export() in your handlers:

/**
 * ---------------------------------------
 * This demo was created using amCharts 4.
 * 
 * For more information visit:
 * https://www.amcharts.com/
 * 
 * Documentation is available at:
 * https://www.amcharts.com/docs/v4/
 * ---------------------------------------
 */

// Themes begin
am4core.useTheme(am4themes_animated);
// Themes end

// Create chart instance
var chart = am4core.create("chartdiv", am4charts.XYChart);

// Add data
chart.data = [{
  "country": "USA",
  "visits": 2025
}, {
  "country": "China",
  "visits": 1882
}, {
  "country": "Japan",
  "visits": 1809
}, {
  "country": "Germany",
  "visits": 1322
}, {
  "country": "UK",
  "visits": 1122
}, {
  "country": "France",
  "visits": 1114
}, {
  "country": "India",
  "visits": 984
}, {
  "country": "Spain",
  "visits": 711
}, {
  "country": "Netherlands",
  "visits": 665
}, {
  "country": "Russia",
  "visits": 580
}, {
  "country": "South Korea",
  "visits": 443
}, {
  "country": "Canada",
  "visits": 441
}, {
  "country": "Brazil",
  "visits": 395
}];

// Create axes

var categoryAxis = chart.xAxes.push(new am4charts.CategoryAxis());
categoryAxis.dataFields.category = "country";
categoryAxis.renderer.grid.template.location = 0;
categoryAxis.renderer.minGridDistance = 30;

categoryAxis.renderer.labels.template.adapter.add("dy", function(dy, target) {
  if (target.dataItem && target.dataItem.index & 2 == 2) {
    return dy + 25;
  }
  return dy;
});

var valueAxis = chart.yAxes.push(new am4charts.ValueAxis());

// Create series
var series = chart.series.push(new am4charts.ColumnSeries());
series.dataFields.valueY = "visits";
series.dataFields.categoryX = "country";
series.name = "Visits";
series.columns.template.tooltipText = "{categoryX}: [bold]{valueY}[/]";
series.columns.template.fillOpacity = .8;

var columnTemplate = series.columns.template;
columnTemplate.strokeWidth = 2;
columnTemplate.strokeOpacity = 1;

// ==================================================

document.getElementById("image").addEventListener("click", function () {
  chart.exporting.export("png");
});

document.getElementById("pdf").addEventListener("click", function () {
  chart.exporting.export("pdf");
});