read

 Ext.Msg.prompt(  
   'Test',  
   'Open the picker',  
   function(buttonId, value) {},  
   me, //scope  
   false, //multiline  
   0, // default  
   { // prompt config  
     xtype: 'selectfield',  
     displayField: 'value',  
     store: 'SettingsTag',  
     usePicker:false  
   }  
 );  

Source

Source 2