minecraft-assets/assets/minecraft/advancements/nether/summon_wither.json

{
    "display": {
        "icon": {
            "item": "minecraft:nether_star"
        },
        "title": "Withering Heights",
        "description": "Summon the Wither"
    },
    "parent": "minecraft:nether/get_wither_skull",
    "criteria": {
        "summoned": {
            "trigger": "minecraft:summoned_entity",
            "conditions": {
                "entity": {
                    "type": "wither"
                }
            }
        }
    }
}