[SalesForce] Pass record id from popup vf page to parent vf page

I have this below vf page where on click a popup vf page is opening where i am creating a new account.
I want on save button of the pop up vf page, the newly created account record id should get pass to parent vf page lookup field.

apex class

public class Accpopup2 
{
    public Account a {get;set;}
    public string status {get;set;}
    public Accpopup2()
    {
      a = new account();  
    }
    public pagereference save()
    {
     Account a = New Account(Name = a.Name, Phone = a.Phone);
     insert a;
     return null;
    }

    public pagereference cancel()
    {
        status = 'true';
          return null;    
    }
}

parent vf page –

 <apex:page standardController="Account" recordSetVar="accs">
    <script>
      function OpenVfpage(pid){
          var newwindow = window.open('/apex/Accsmallpopup?id='+pid, 'name=_blank','height=500,width=500,left=250,top=100'); 
      newwindow.focus();
     }
    </script>
        <apex:form >
            <apex:pageBlock >



    ParentAccount : <apex:inputfield value="{!account.ParentAccount__c}" label="ParentAccount"/>

                        <apex:commandButton value="Click" onclick="OpenVfpage('{!account.id}')" reRender="two"/>

            </apex:pageBlock>

        <script>
    function processAccount(accountId){
        console.log('Inside Parent page --> AccountId passed from popup is-->'+accountId);
    }
</script>
        </apex:form>
    </apex:page>

Popup vf page –

<apex:page controller="Accpopup2" showHeader="false" sidebar="false" id="the">
   <apex:form id="page">
    <apex:pageblock id="close">
    <apex:pageBlockSection >
        <apex:inputfield value="{!a.Name}"/>
         <apex:inputfield value="{!a.Phone}"/>
     </apex:pageBlockSection>
        <apex:pageblockbuttons >
        <apex:commandButton value="save" action="{!save}" reRender="dyanamicJs"/>
        <apex:commandButton value="cancel" action="{!cancel}"/>
            </apex:pageblockbuttons>
            <apex:inputHidden id="pass" value="{!status}"/>
             </apex:pageblock>

            <script language="JavaScript" type="text/javascript">
            if(document.getElementById('the:page:close:pass').getAttribute('value') == 'true')
                {
                    window.top.close();
                }
            </script>   

        <apex:outputPanel id="dyanamicJs"> 

window.opener.processAccount('{!a.Id}');