I am assuming following has already been done:
1) my_connection_handle = cyBle_connHandle
2) You are enabling notification from CySmart Windows by writting a 0x0001 (or in gui, 01:00) to the CCCD of the corresponding characteristic to which you want the notification.
3) the handle_val_pair_adc.attrHandle is equal to the attribute handle on the Characteristic you want to write to. This value can be foung in ble_custom.h
Associated application note for handling custom services/characterisitc is here, in case you want to refer:
my_connection_handle = cyBle_connHandle was the crux!
Thank you so much!