You want to put that between your freshwater tank and the pump.

Here is my install.
Before, Hose from freshwater tank to the Left side (input) of pump.

After, Hose from freshwater goes to T and T goes to inlet of pump. You can see the clear fill hose just sits there (left side of pic) until I'm ready to use it.

You may notice an additional hose for ease of connection. That came from the HW store. Its a bathroom sink/toilet hose. 1/2" I believe... Someone this site gave me that tip and it worked great!
Do you have a bypass setup for your hotwater tank? You should install that as well so you don't fill your HW tank up with antifreeze.