Flutter Firestore add new document with Custom ID

String uniqueCode = //Your Unique Code
DocumentReference reference = Firestore.instance.document("test/" + uniqueCode );

//Setting Data
Map<String, String> yourData;
reference.setData(yourData);

You can use set() function instead of add().

Here's full code:

final CollectionReference postsRef = Firestore.instance.collection('/posts');

var postID = 1;

Post post = new Post(postID, "title", "content");
Map<String, dynamic> postData = post.toJson();
await postsRef.doc(postID).set(postData);

I hope that help anyone.

Update 2021:

Instead of using add, use set on the document.

var collection = FirebaseFirestore.instance.collection('collection');
collection 
    .doc('doc_id') // <-- Document ID
    .set({'age': 20}) // <-- Your data
    .then((_) => print('Added'))
    .catchError((error) => print('Add failed: $error'));