Is your pool builder a "she"? If so, I think I know which company you are talking about. We used them to build a few years ago and we are getting ready to start a new pool with them (we moved to a new house). Feel free to message me.
Our overflow drain is a threaded PVC pipe behind the grate. Home Depot sells a threaded plug for maybe $3? I screw it into the pipe and replace the cover. When we are going to be gone and/or during the winter I remove the plug.