How to reload a stateless form with updated data

I have a stateless form with 2 fields. One is a key of a form. The user enters that field and clicks a button on the stateless form. This button retrieves the form using the key, and copies a data-filed from the retrieved form into the second field on the stateless form. What do I need to do to show this second field to the user?

The <reload> function basically refreshes everything.

Any suggestions?

thanks

Henk