You need to activate “heart-rate sharing”.
For runkeeper, before logging starts, go to bluetooth setting page of your P2 watch.
You should see a prompt to start sharing HR. Once granted, you should see a red
heart mark on the watch icon of runkeeper. Here you go. Don’t forget to turn it off in the
bluetooth menu of your watch after your work out finished.
Another app you can try is JayPS for pebble (both watch and phone app). For Strava,
I never manage to make it work for HR.