Could not get value from controller using command object

This my GSP form

 <g:form action="saveDP" method="POST">
        <input type="hidden" value="${this.agent}" name="agent">
        <g:submitButton name="create" class="btn btn-success" value="Add" />
 </g:form>

This my controller

def saveDP(DepositoryParticipant depositoryParticipant) {
    println "agent = " + depositoryParticipant.agent
}

This is from source html

<form action="/cds/agent/saveDP" method="post" >
        <input type="hidden" value="Depository Participants" name="agent">
        <input type="submit" name="create" class="btn btn-success" value="Add" id="create" />
 </form>

This is result in console

agent = null

I am trying to use command object. The value is null. How to get value?