Recognize the left click. Any help?

I am editing the turbo fusion script to make a really cool knife. Everything works awesomely so far :3 Now all I need if the activated knife to work when left clicked. Right now it works by pressing the "f" key. Is there a 'string.char()' number that I could use to recognize it?

Here is the line I am editing:

KnifeKey = "f";

that specific line is connected to the entire script to that when you press "f" it knifes.

There is another line that uses the shift to sprint. As an example of the string.char()

SprintKey = string.char(48);