One PFS design "flaw" (intentional or not), made it impossible to connect the PF IR Receiver to any old 9V source by simple using any of the PF Extension Wires (8886, 8871).
Thus several workarounds have been proposed and this was already discussed here at TBs before [1, 2]. From powering the IR Receiver from one of its outputs, to use an empty PB battery box there is always a solution that most fits your needs.
However these days while preparing a post still to come, I found the most compact workaround and probably the most interesting one. I've seen it being used in a mini tank MOC [3*, 4], by Jaeho Jung from BrickInside (the largest LUG in South Korea).
According to Jaeho Jung, this is a method developed by Minshik Kong, aka Sarafiel [5, 6] (another BrickInside member). The method explanation itself, can be found here* (you should be able to recognize the first video there ).
In simple words, the method consists of using a thin aluminum foil to short-circuit the 9V/C1 and 0V/C2 pins on the PF lead that connects with the legacy 9V source, via any 9V/PF adapter/extension cable.
I'm sure the images below, from Philo's 'Power Functions™ Presentation' webpage, will help you to understand the principles behind this idea.
Simple, brilliant and no bulky or limiting solutions, I would say!
*) All the links with reviews and uses of this method, are written in Korean but hopefully the photos seapk by themselves.