custom_mc_advancements/data/canopy/advancements/end.json

{
    "display": {
        "icon": {
            "item": "minecraft:end_portal_frame"
        },
        "title": "The End.",
        "description": "That was easy.",
        "frame": "task",
        "show_toast": false,
        "announce_to_chat": false
    },
    "parent": "canopy:root",
    "criteria": {
        "in_end": {
            "trigger": "minecraft:location",
	    "conditions": {
                "dimension": "minecraft:the_end"
	    }
        }
    }
}