if your sandbox is a full sandbox then there is no problem... by using the static id there....

we have full sandbox but this is not works for me because we had new fields and object created in sandbox after we refresh from production, when we deploy the new fields and object from sandbox to production, the Id was changed.

I had workwround -- but it need to be update on every sandbox refresh

public string getRefOppsId(){
		   	Schema.DescribeSObjectResult d = Schema.SObjectType.Reference_Opportunity__c;
			system.debug('======DescribeSObjectResult'+d.getKeyPrefix());
			if (d.getKeyPrefix()=='a0a'){
				//Sandbox Id
				return 'a0Q';
			}
			else {
				//Production Id
				Schema.DescribeSObjectResult d1 = Schema.SObjectType.PXRB_Opportunities__c;
				return d1.getKeyPrefix();
			}
	   }

	   public string getRefOppsFieldId(){
		   	Schema.DescribeSObjectResult d = Schema.SObjectType.Reference_Opportunity__c;
			system.debug('======DescribeSObjectResult'+d.getKeyPrefix());
			if (d.getKeyPrefix()=='a0a'){
				//Sandbox Id
				return 'CF00N500000025aBy';
			}
			else {
				//Production Id
				return 'CF00N500000025aBy';
			}
	   }

<apex:commandButton style="align:center;" value="Create new SFDC Opportunity" onClick="window.open('/{!RefOppsId}/e?{!RefOppsFieldId}={!PXRB__c.Name}&{!RefOppsFieldId}_lkid={!PXRB__c.Id}&saveURL=%2F{!PXRB__c.Id}&retURL=%2F{!PXRB__c.Id}');"/>

 this will not required manual change the ID when deploy the changes from sandbox to production, only need update the code whenever sandbox get refreshed.

thank you for the idea, this is basically i did in apex class with the static id, determinate the difference between sandbox and production.

Qingsong

then y can't u create a simple vf page to open a pop up for this scenario...