How can I correctly display utf-8 encoded characters in a MessageBox in C++?

I used mppoker/cpp-httplib

bool Request::Login(){    std::string json = "{ \"email\": \"" + std::string(User::GetUsername()) +"\", \"password\": \"" + std::string(User::GetPassword()) +"\" }";    auto res = client->Post("/api/client/auth/login", json, "application/json");    if (res && res->status == 200) {        auto j = nlohmann::json::parse(res->body);        // perty print        int indent = 4;        std::string s = j.dump(indent);        return true;    }    else {        auto j = nlohmann::json::parse(res->body);        std::string err = j["error"];        MessageBox(NULL, err.c_str(), "Error", MB_OK);        return false;    }}

Example API Response

{"statusCode": 400,"error": "`Email sai định dạng`""path": "/api/client/auth/login","method": "POST"}

The code I provided is returning an error message from an API in json format, but when trying to display the error message in a MessageBox, the utf-8 characters are not being displayed correctly. What can I do to properly display utf-8 encoded characters in a MessageBox in C++?