alltalk_tts/system/tts_engines/new_engines.json

{
    "engines_available": [
        {
            "name": "parler",
            "selected_model": "these are examples of how you add new engines into the list here. You would put a model name in this line"
        },
        {
            "name": "xtts",
            "selected_model": "xtts - xttsv2_2.0.3"
        }
    ]
}