Hello Bikash
Make sure you define the default method of BOR in SWO1 in "Defaul" Tab. Usually it is a "display" method of BOR object but you can define your own BOR method.You will need to instantiate the BOR object and bind it to the User decision task in WF template.
When the approval task will appear on UWL, you will see the link . If you click the link, it will execute the code in "DISPLAY" method(or default method maintained ).
Alternative approach is "Secondary methods" . You can add BOR method in "methods" tab of decision task and it will open a new session when workitem is executed on UWL . Refer Detail below
Maintenance of Tab Page Methods - Roles in Workflow - SAP Library
Secondary Methods, Before and After Methods - ABAP Development - SCN Wiki
Hope it helps
Regards
Sandy