How to populate the result of a $lookup

I have 3 collections, posts, comments and users, the schemas for posts and comments are below:

post schema

const PostSchema: Schema = new Schema({
    author: { type: String, ref: 'User' },
    postID: {type: String},
    text: { type: String },
});

comment schema

const commentSchema: Schema = new Schema({
    author: { type: Schema.Types.ObjectId, ref: 'User' },
    parentPost: {type: Schema.Types.ObjectId, ref: 'Post'},
    parentPostID: {type: String}, // Post that was commented on
});

To get the comment from each post, I do an aggregate query:

 const aggregateQuery = [
                            {
                                $match:{postID:{ $in: postKeys},},
                            },
                            {
                                $lookup:
                                    {
                                        from: 'comments',
                                        localField: '_id',
                                        foreignField: 'parentPost',
                                        as: 'Top_Comment',
                                    },
                            },
                            {
                                $sort: {
                                    createdAt: 1,
                                }
                                ,
                            },
                        ]

That query works, but I can't figure out how to populate the author property of the comment since its a ref to a user.

What I tried

I tried using pipeline, but I'm a new to aggregation so I didn't know how, any ideas and solutions would be really appreciated