Bring up the Control Center in iOS on your iPhone. (Method depends upon the model and iOS version. Generally, swipe down from the upper right corner.)

Locate the Low Power Mode button, and tap it.

Enable the "Charge past 90%" option now available with the MagSafe Battery Pack attached.

Tap the screen to dismiss the popup, then dismiss the Control Center.

For more information on Low Power Mode, see the Apple KB article.