You are not going to get adequate threads in that.
It would sure help if you could get those of us who lack mind-reading capability, at least a little more than a hint of what you are trying to do, because perhaps this might rule out some of the many options that you have.
Rivnuts.
Weldnuts. (Used in automotive all the time. Commonly secured using projection-welding, resistance-welding, "spot-welding".)
Through-hole both parts and use screws and nuts (and washers), and forget trying to thread the metal.
Through-hole and J nuts. (Google it.) Works if the attachment point is close to the edge of the plastic part and the thickness of the part is in an acceptable range. Used for securing motorcycle fairings to underlying metal frames all the time.
Through-hole the sheet metal and thread the plastic piece, using self-tapping fasteners similar to wood screws, or possibly actual wood screws, depending upon the consistency of the plastic that you are trying to attach. Requires the screw to go in from the other side.
There are lots of ways to do this. More information = more helpful answers.