borderRadius

Specifies the corners smoothing value applicable both for the border of the series element being selected with the mouse and the element itself.

seriesSettings: {
    rangebar: {
enablePointSelection:true,
        activeSettings: {
            pointSelectedState: {
                enabled: true,  
                borderColor: 'green',   
                borderWidth:3,
                borderRadius: 5
            }
        }  
    }
}

This setting is demonstrated in the following example.