Use a C# method to insert data into a SQL table with a column set as Uniqueidentifier type fails

Having initially tested in Sql the following command INSERT INTO UserActivationDB (UserId, ActivationCode) VALUES (3,NEWID()) that works perfectly, I have been attempting to adjust a C# method to circumvent an Exception.

private void Checkmail(int userId)
        {
            string constr = ConfigurationManager.ConnectionStrings["RegisterDB"].ConnectionString;
            string activationCode = Guid.NewGuid().ToString();
            using (SqlConnection con = new SqlConnection(constr))
            {
                //using (SqlCommand cmd = new SqlCommand("INSERT INTO UserActivationDB VALUES(@UserId, @ActivationCode)"))
                using (SqlCommand cmd = new SqlCommand("SET IDENTITY_INSERT UserActivationDB ON INSERT INTO UserActivationDB VALUES(@UserId, NEWID()) SET IDENTITY_INSERT UserActivationDB OFF"))
                {
                    using (SqlDataAdapter sda = new SqlDataAdapter())
                    {
                        cmd.CommandType = CommandType.Text;
                        cmd.Parameters.AddWithValue("@UserId", userId);
                        //cmd.Parameters.AddWithValue("@ActivationCode", activationCode);
                        cmd.Connection = con;
                        con.Open();
                        cmd.ExecuteNonQuery();
                        con.Close();
                    }
                }
            }
        }```

**Error message**

```System.Data.SqlClient.SqlException
  HResult=0x80131904
  Message=An explicit value for the identity column in table UserActivationDB can only be specified when a column list is used and IDENTITY_INSERT is ON.
  Source=.Net SqlClient Data Provider
  StackTrace:
<Cannot evaluate the exception stack trace> 

Ps: As I was stuck in keeping activationCode as string and inserting it into a uniqueidentifier type in the UserActivationDB table (merely 2 columns) , I 've commented it and substitute it with NEWID() but I am still encountering similar issues at cmd.ExecuteNonQuery();

Thus a feedback on such issues, particularly on the cmd = new SqlCommand(...) object, would highly be appreciated.

Best,