That’s the right command but it doesn;t make the boolean work.
Your problem stems from the fact that your model is out of tolerance.
I suggest not using booleans because you end up wasting enormous amounts of time when they fail and they fail often. If you do the operations manually you will quickly discover where the problem is
You can either loosen your tolerance or just extend the cutting surface upward so that the out of tolerance edges don’t mess things up.
Here is how I set it up for trimming and joining.
Chargerx.3dm (523.9 KB)
Notice I eliminated the surfaces that are not part of the solution.
Then just select both objects and run Trim and click on the parts that get trimmed off. Then run join.