[chirp_users] BF 888S - side key function - Transmit Power

I am using daily build 2017126 to programme a standard baofeng BF888S. I can change the individual channel power settings through the Power column in the tabular Memories screen which is great. I have verified this actually works with a power meter. However I can't re-designate the Side Key Function as Transmit Power via the Settings screen. It does re-designate that key to the other functions when selected. Is that a software bug or operator error on my part ?