Wordpress TinyMCE add a description to a popup form
After your select list, you can add a container and put html in it.
editor.windowManager.open( {title: 'Container',body: [{ type: 'listbox', name: 'style', label: 'Style', 'values': [ {text: 'Clear', value: 'clear'}, {text: 'White', value: 'white'}, {text: 'Colour 1', value: 'colour1'}, {text: 'Colour 2', value: 'colour2'}, {text: 'Colour 3', value: 'colour3'}, ]} /*add in the following with the comma */ ,{ type: 'container', html: '<p>Enter your Text here</p>' }],onsubmit: function( e ) { editor.insertContent( '[container style="' + e.data.style + '"]<br /><br />[/container]');}