You will need a USB-C to Lightning cable.  Simply plug the USB-C side into the Turtlebox underneath the port flap where it says "OUT".  Plug the lightning end into your iPhone.  Please note that your Turtlebox is not waterproof when the port flap is open so take care that water does not get near the ports.  Here is a link to the cable on apple.com:  

https://www.apple.com/shop/product/MM0A3AM/A/usb-c-to-lightning-cable-1-m?fnode=e975ab9a636fb4bfc2abd24f3df3cb543e3dca99902fa8399a73c4a2f73e154b599d31103ded591191153af8ff2ddf3bfdfbbee490c73d350fbb20d6424db40e5834ca6071d4e58a8c9fddd3da5edfa316e585e4be332071ae3e65b0d159d798a09e3bcab3c78502ab32456da001c3d3