There is no feedback parameter – you need to put a mixer before the shifter, with an input connected to the shifter's output – controlling the amount of output that is mixed in with the input will control the amount of feedback.
But, I may be misleading you here – this will give a kind of "crystals" sound, while you may want just a sustained loop. If the second is the case, you could use a DELAY module, again with a mixer to give the feedback. Or, start with a simple looper preset and add the shifter.
