You are not able to record Power with Garmin's Finish Time.
If you are using Garmin's Finish Time in the native Run Mode, it will open a new screen showing your distance remaining, estimated finish time, and average pace. You are not able to edit the screen to show your ConnectIQ Data field to record Power.
This means when you start your distance target: Power will not be recorded and your finished run will not import into PowerCenter.
If you need more assistance, contact firstname.lastname@example.org.