Set the front AF dial to manual and you are set to use the back button focus.
Please note that using "Instant AF=AF-C" is probably not a good idea on the X-T1 (and other 16-mpix Fujis). If you press-and-release to simulate AF-S, the camera may not complete the AF operation and can leave the lens misfocused. If you keep it held while shooting a burst, focus is locked with the first frame and stops tracking. And those two are pretty much the main reasons you'd use BBF with AF-C.
With BBF you sacrifice the "full press without stopping at half-press" technique for capturing single shots of a moving subject. On the 16-mpix cameras, that's generally more reliable than using AF-C.
I've also noticed some hard-to-pin-down squirreliness when using back-button AF on 16-mpix Fujis. That seems to be tied to the fairly complex logic of how the sensor is to be shared among the viewfinder, metering, autofocus, and image capture processes. Adding more buttons and combinations of locking sequences seems to get it confused at times. Here again, my sense is that it's more of a problem with AF-C than with AF-S.
The whole thing was completely redesigned for the 24-megapixel cameras, and seems much more robust.
If you're working with moving subjects, my personal recommendation is not to use BBF on the 16-megapixel Fujis. If you only do stationary subjects and use Instant AF=AF-S, it's probably okay.
