JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://dev.sencha.com/deploy/dev/resources/css/ext-all.css">
<script src="http://dev.sencha.com/deploy/dev/adapter/ext/ext-base.js"></script>
<script src="http://dev.sencha.com/deploy/dev/ext-all.js"></script>

JavaScript

Ext.ns('Ext.ux.form.HtmlEditor');

Ext.ux.form.HtmlEditor.HR = Ext.extend(Ext.util.Observable, {
    init: function(cmp){
        this.cmp = cmp;
        this.cmp.on('render', this.onRender, this);
    },
    onRender: function(){
        this.cmp.getToolbar().addButton([{
            iconCls: 'x-edit-custom', //your iconCls here
            handler: function(){
                this.cmp.insertAtCursor('<hr>');
            },
            scope: this,
            tooltip: 'horizontal ruler',
            overflowText: 'horizontal ruler'
        }]);
    }
});

var w = new Ext.Window({
    width: 550,
    layout: 'fit',
    items: [{
        xtype: 'htmleditor',
        plugins: [new Ext.ux.form.HtmlEditor.HR()]
    }]
});
w.show();