Help. C# Windows Form Login using mongodb

Working with Data Drivers
1

Here is code for the registration. how to code in login to get data username that matching with the password

public class UserReg
    {
        public string userName { get; set; }
        public string password { get; set; }
        public string role { get; set; }

    }

    private void btnRegister_Click(object sender, EventArgs e)
    {
        var client = new MongoClient();
        var db = client.GetDatabase("Project_1");
        var collection = db.GetCollection<UserReg>("UserReg");

        UserReg userReg = new UserReg();
        userReg.userName = txtUsername.Text.ToString();
        userReg.password = hc.HashPass(txtPassword.Text.ToString());
        userReg.role = cboRole.Text.ToString();

        collection.InsertOneAsync(userReg);

    }