Get value of pl/sql-generated form element?

 All,

I have an APEX page with a PL/SQL dynamic region.  This regions calls a PL/SQL procedure that generates html.  In this html, a form is opened with a text box (id="txtUserEnteredText") awaiting user input.  I need to know how to set the value of an APEX page item on a button click with the user-entered text. What I have tried is the following dynamic action for the button.

-- Dynamic Action

Event: On Click

Section Type: Button

Button: P20_GET_TEXT

-- Action

Action: Set Value

Set Type: JavaScript Expression

JavaScript Expression: $x(document.getElementById('txtUserEnteredText').value);