How do I attach a weed wacker to a bicycle

That would be ridiculously dangerous, you shouldn't bother trying to do it, and even if you could, you wouldn't be able to use the bicycle and the weed whacker at the same time.

It is rediculously dangerous, when I was Younger, I did it all the time, At 50, I still ride a
6' Unicycle.
I haven't tried what you propose, but I would remove outer plastic if necessary to find a good place to bolt onto, Have seen it done with a little roller wheel that rotates against the tire(In this case engine needs to rotate opposite direction of wheel, to spin wheel forward.
A Chain, or Pully and belts.
You will likely have to change size of wheel, pullys or gears to work efficiently.
Good Luck