You can't ground an extension cord with a two-prong adapter.

That’s not how it works. A two-prong adapter (often called a “cheater plug”) is designed for two-prong outlets, not for adding grounding to an extension cord. The little metal tab on the adapter is meant to be screwed into the center screw of a grounded outlet cover plate, which itself has to be properly grounded. That screw path is already sketchy—adding an extension cord into the mix breaks the ground path entirely.

If you need to use an extension cord with a grounded appliance, use an extension cord that has a three-prong plug and plug it into a properly grounded three-prong outlet. If you only have two-prong outlets, the real solution is to have an electrician update the outlet or use a GFCI adapter (which provides shock protection but not a true ground). A cheater plug on an extension cord is just a fire hazard waiting to happen.

Don’t hack it. Get the right cord or fix the outlet.