I have a command link that opens a popup window to create a related record from the opportunity.  How can I pass the opportunity into the lookup field on the related item

this is the command button to open the popup

<apex:commandlink styleClass="btn" style="text-decoration:none;padding:4px;" onclick="javascript:void window.open('https://c.na35.visual.force.com/apex/createTest','1478265075053','width=700,height=500,toolbar=0,menubar=0,location=0,status=1,scrollbars=1,resizable=1,left=0,top=0');return false;">New Buying Center</apex:commandLink>

Here is the visualforce page to create the related record

<apex:page standardController="Buying_Center__c" showHeader="false" sidebar="false">
    <apex:form >
   <script language="JavaScript" type="text/javascript">
function CloseAndRefresh(){
window.opener.location.href="/{!$CurrentPage.parameters.id}";
      window.top.close();
      
  }
</script>

        <apex:pageBlock >
        <apex:pageBlockButtons location="bottom">
        <apex:commandButton action="{!save}" Value="Save" oncomplete="javascript&colon;CloseAndRefresh()" />
        </apex:pageBlockButtons>
     
            <apex:pageBlockSection title="Create New Buying Center">
                <apex:inputField value="{!Buying_Center__c.opportunity__C}"/>   
                <apex:inputField value="{!Buying_Center__c.Influence__c}"/>
                <apex:inputField value="{!Buying_Center__c.Urgency__c}"/>
                                         
            </apex:pageBlockSection>
        </apex:pageblock>
    </apex:form>
</apex:page>