Help. C# Windows Form Login using mongodb

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();