something entered in text box, display a add button next to it

I have a text box that I can only allow numeric characters to be entered into it. Once the user has entered something, I'd like to "display" an add button that will allow me to add this record to a DB. How can I hide button the first or "disable" them (grey them out) and once I have something entered in the text box, make the button active to my user?

I'm thinking I probably have to do something with the onkeypress event?

I just need a snippet of code to understand it. Thanks in advance.Start Free Trial