firebase firestore update variable nested data at runtime

How can I update a nested value that is undetermined until runtime?

For example, I wish to update the value at the key which is a user's id. I don't know which user to update until runtime.

The docs give this example code (https://firebase.google.com/docs/firestore/manage-data/add-data#update_fields_in_nested_objects):

db.collection("users").doc("frank").update({
    "age": 13,
    "favorites.color": "Red"
})

I have modified it using template literals to suit my purposes:

db.collection("chatGroups").doc(route.params.data.id).update({
    `markedDates.${auth.currentUser.uid}`: markedDates
});

but it throws syntax errors.