Have textbox which is hidden initially. Something like...
Now with the "onClick" event, associate a JS snippet like...
This will give you the desired behaviour at the client end. To retrieve the value of this textbox in your action class, have the getter/setter => [getTxtVal()/setTxtVal()] : in your Action form class [incase you're using
Struts] or else have the getter/setter in your Action class itself [if using Struts2]