If it's a dynamic form, you can set a default value in struts-config.xml:

<form-bean      name="contactForm" 
                  type="edu.example.ContactForm"> 
      <form-property name="onHoldFlag" 
                               type="java.lang.Boolean" initial="true"/> 
  </form-bean> 

Otherwise, the place to do this is in your Action, before you forward to the
JSP. You only need to set the default the first time  the form is displayed.
Exactly where and how depends on how you've written  the app