The best way is to do this at your router, if it supports "Address Reservation". Find the HS100 under its current IP address in your router's Client List, and note down the MAC address. Now navigate to the router's Address Reservation menu and enter the MAC address and the required IP address. Whenever the HS100 reboots (eg when it is powered off and then on) it will be given this same address.

Doing it this way doesn't require any reconfiguration at the HS100 (if this is even possible).

Not being able to set a fixed IP address is going to be a problem for many people. I understand that the focus is all about the Kasa app, but surely the app could discover devices with a fixed IP address.

you can send this as suggestion to tp-link email support. They can consider this function in the next update for kasa app