In a procedure, how do I check if an item is in the main hand?

I am making a mod with a spear in it that you can dash into enemies with when pressing C, but I can't figure out how to make it so that it only works when I have the spear in my MAIN HAND.

This is how I want the keybind to work:

1. Key is pressed
2. Player dashes forward
3. Enemy player dashed at takes damage
4. Player stops dashing

If anybody could help that would be great!

Thanks!