Help creating dynamic forms

[v1973]

how to do i add new elements to form panel.
for e.g i want to add checkbox on a button click.

contract_add is form panel
addPlant= function(formPanel,scope){
plant_chk_bx={xtype:'checkbox',fieldLabel :'',name:'added_plant_' + Ext.id()};
contract_add.getForm().add(plant_chk_bx);
contract_add.getForm().show();
contract_add.render('contract-panel');

Please help.

Thanks,
v