The transfer function instrument in SpectraFoo can be found under the WINDOW tab on the top of the app window.

From there you need to turn the instrument on:

To use the transfer function tool in Foo efficiently, it helps to learn the key commands by heart. This will save a lot of time!

This is the default list of key commands:

These key commands relate directly to the transfer function window:

CONTROL / C = CLEAR
CONTROL / F = RUN / STOP
CONTROL / S = SAVE SNAPSHOT
CONTROL / T = COMPUTE DELAY

These key commands are universal to all instrument windows but are very useful for managing the transfer function window:

CONTROL / G = TOGGLE SIGNAL GENERATOR ON/OFF
CONTROL / A = TOGGLE FRONT INSTRUMENT CONTROLS

This is the instrument control window in the TF instrument.

Another way of opening and closing it is to click on this button:

If you forget them , you can reference the same list located here from within Foo: