You have to add focus and blur listeners to the textfield.

Inside the focus event the first argument is the textfield. The text you add to the window is inside the textfield.popupText.

EXAMPLE

    xtype: 'textfield',

    popup: null,
    popupText: 'Testing Popup Text',

    listeners: {
        focus: function(field) {
            field.suspendEvent('blur');
            field.suspendEvent('focus');

            field.popup = field.popup || Ext.create({
                xtype: 'window',
                ownerCt: field,
                height: 80,
                width: 200,
                html: field.popupText
            });
            
            field.popup.showBy(field.el, 'l-r')

            field.focus();
            field.resumeEvent('blur');
            field.resumeEvent('focus')
        },
        blur: function(field) {
            field.popup.destroy();
        }