1. Sir, so far as I know, there is no such prescribed form for surrendering sub-code number as per procedure of ESIC.
2.. The question is why you want to surrender such sub-code ? Whether the Branch in respect of which said sub-code was allotted has been closed permanently ?. If so, you can inform the appropriate Regional/sub-Regional Office and Branch Manager of ESIC that your such Branch has been closed. This is required in order to ensure that your Branch may not be treated as defaulter when such offices will not received any compliance in respect of your said Branch.