Hi

Try the below code snippet as reference:

------------- vf page--------------

<apex:page standardController="opportunity" extensions="myControllerExtension">

    <apex:form >

        <apex:pageBlock > 

           <apex:pageBlockSection title="Information Avenant">

                    //to display the name of the master opportunity and can edit

                  <apex:inputField value="{!Opp .name}"/> <p/>

                 <apex:inputField value="{!Opp .closedate}"/> <p/>

                 <apex:inputField value="{!Opp .StageName}"/> <p/>

<apex:outputField value="{! Opp.N_police_d_origine__c}"/> <p/>
                  <apex:inputField value="{! Opp.Date_d_avenant__c}"/> <p/>
                  <apex:inputField value="{! Opp.Statut_Police__c}"/> <p/>   

              </apex:pageBlockSection>  

             <apex:pageBlockButtons >

                   <apex:commandButton value="Enregistrer" action="{!enregistrer}"/>

                   <apex:commandButton value="cancel" action="{!cancel}"/>

             </apex:pageBlockButtons>

                           </apex:pageBlock>

    </apex:form>

</apex:page>

-------------------------------- apex controller -----------------------

public class myControllerExtension {

private String name;

   public Opportunity Opp {get;set;}

   public Opportunity OppAcc {get;set;}

   ApexPages.StandardController controller;  

   public myControllerExtension(ApexPages.StandardController c)

   {

          Opp = (Opportunity)c.getRecord();

   }

        public PageReference Avenant()

      {

        return Page.Avenant;

      }

     public PageReference cancel()

     {

            PageReference opportunityPage = new ApexPages.StandardController(opp).view();

            opportunityPage.setRedirect(true);

            return opportunityPage;

     }

   public PageReference enregistrer() {

   insert Opp ;

   return null;

//add a news code to crate a new opportunity

}

}

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