Mongocxx - How to update an array within a document? [C++]

Working with Data
1

Hi everyone, happy to join this forum!
I have a MongoDB document with the following structure:

{
"_id":"$oid":"621fbaeaeedd1c000e60fbd2"},
"username":"myuser",
"password":"mypassword",
"comments":["comment1", "comment2", "comment3"]
}

I have a vector of comments:

std::vector<std::string> mycomments; 
mycomments.push_back("comment2"); 
mycomments.push_back("comment4");

I would like to insert, if it is not already present, each string of the vector “mycomments” into the array “comments” in MongoDB document. I think that I should use the update_one function.
I would like to obtain this final result:

{
"_id":"$oid":"621fbaeaeedd1c000e60fbd2"},
"username":"myuser",
"password":"mypassword",
"comments":["comment1", "comment2", "comment3", "comment4"]
}

I tried to do it but I don’t know how to proceed:

mongocxx::collection coll = db["users"];
coll.update_one(
            document{} << "username" << username
                       << finalize,
            document{} << ???
                       << open_document
                       << "comments" << ???
                       << close_document << finalize);

Thanks in advance.