When you say you cannot see it in the available options, do you mean that it is not there to select?
I asume you have opened the Control Panel and gone to Keyboard Properties? You should see a drop down menu on the top left for the key assignment. If you want to use the timeline / master viewer. There are various tabs, but select FX and there is a symbol looking like a curled up arrow about 6th in from the left. This is the 2D CPU editor.
You should then be able to drag this to a spare key of your choice.