Dear Riyaz,
In standard MSS add on role under the Work overview workset you will have service called Time approval and task reminder.
Clicking on the service the POWL will be opening where the end user can approve/reject the leave request/CICO etc.
Hope you are using standard workflow id- WS21500001 for the leave request approval.
Also in case if you want to change the standard POWL you can do the required settings under the path SPRO-> Personnel Management->Manager Self-Service (Web Dynpro ABAP) ->Personal Object Worklist ->Cockpit for POWL Administration
Regards,
Mithun K