Proper way to bypass heater?

If you tap into the factory sender circuit;
I fear your device may alter the reading on your gauge, which is designed to work with a variable resistance over a distinct range. By adding a different resistance device, that device presents as a short, and depending on the degree of the short, the gauge will read and respond to it.
My guess is that the company has thought of that, and mentions it in their instructions, or supplies a new sender; IDK
In any case, I suppose you have proved your gauge with an IR gun ? ................