Not able to Populate lookup field using custom button

Question

I am trying to populate lokup field thorugh custom button.I am using on click javascirpt.

code:

window.open(a5S/e?CF00N18000000esbp={!Study__c.Name}&CF00N18000000lA6S_lkid={!Study__c.Primary_Product_Id__c})

I am getting the error URL No Longer exists on clicking custom button. also when I click on custom button which has the logic salesforce url changes to

https://cs23.salesforce.com/servlet/a5S/e?CF00N18000000esbp=S-0265&CF00N18000000lA6S_lkid=01t18000000w7Ge

Answer 1

Finally Got it.

/a5S/e?CF00N18000000esbp={!Study.Name}&CF00N18000000lA6S={!Study_CELG_IPS__c.Primary_Product__c}