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You can use the plot line functionality of highcharts.
plotLines: [{
color: 'red', // Color value
dashStyle: 'longdashdot', // Style of the plot line. Default to solid
value: 3, // Value of where the line will appear
width: 2 // Width of the line
}]
The exact link is-
https://www.highcharts.com/docs/chart-concepts/plot-bands-and-plot-lines