series.legendItem.highlight.markers.borderRadiusNumber

The border radius in pixels when type is set to "roundedRect".

Example - set the legend item highlight markers border radius

<div id="chart"></div>
<script>
$("#chart").kendoChart({
  series: [{
    type: "line",
    name: "Series A",
    data: [1, 2, 3],
    legendItem: {
      highlight: {
        markers: {
          type: "roundedRect",
          borderRadius: 10
        }
      }
    }
  }]
});
</script>