ExtJS 6.2: Add afterLabelTextTpl Dynamically

I'm trying to dynamically add an icon after various fields to be used as help (i.g. when the user hovers over the icon a tooltip will appear). I was trying to do this using the afterLabelTextTpl config in a beforerender listener, but I'm not having any luck. Can anyone suggest how I might set afterLabelTextTpl dynamically or another/better method?

This should work with classic toolkit:

Ext.application({
    name: 'Fiddle',
    launch: function () {
        Ext.create('Ext.form.Panel', {
            title: 'Form',
            bodyPadding: 8,
            defaultType: 'textfield',
            items: [{
                xtype: 'textfield',
                fieldLabel: 'Field label',
                listeners: {
                    beforerender: function (field, eOpts) {
                        field.afterLabelTextTpl = [
                            '<tpl>',
                            '<span title="Tooltip" style="cursor: pointer"; class="x-fa fa-user"></span>',
                            '</tpl>'
                        ];
                    }
                }
            }],
            renderTo: Ext.getBody()
        });
    }
});

Setting cursor: pointer is optional but it might be useful. You can add additional styling (padding etc.) in the style part of the span.