Im quite surprised the Public Works Dept gave you a go ahead to tap into their line without telling you the way it will be done, and what they will inspect, especially if your town has progressed to having a separate “Drainage Dept”. I cant picture them turning loose a DIY on their pipe with a sawzall. There are tapping tees for this, just not at Home Depot. Call back the the drainage department and ask what they require to make the tap.