Customizing default y-axis label in Highcharts

I want to prefix a $ to the default y-axis label. My bar chart is using values in the millions so the chart is returning value-MM (80MM, 30MM). What I would like to do is format the y-axis like $-value-MM ($80MMm $30MM). I have tried the code below and can't get it to work?

yAxis: [{ // Primary yAxis
    labels: {
        formatter: function () {
            return '$' + this.value;
        }
    },
    title: {
        text: 'Revenue',