How to add a parameter to the existing HttpServletRequest of my Java Servlet?

I want to add a new parameter to the parameter map of my HttpServletRequest.

The following code

 request().getParameterMap().put("j_username", user);
 request().getParameterMap().put("j_password", pwd);

creates this error

no modifications are allowed to a locked parameter map

What is the correct way to do this?