有些事如何做: How to make a keybind that makes left mouse button held down

So, what I've tried to do is to make a code while the key "a" is pressed the left mouse button is held down, but if "a" is not pressed left mouse button is not held down anymore. I've tried so many options, but I haven't succeed yet. If you can make it beginner friendly I would appreciate that, but if you can't, just post it anyway :)