Question

I want to manually create a ID field where I do MAX + 1, and I want to do it in one QUERY so I am certain 2 entries cant get the same field.

        using (MySqlConnection dbConn = new MySqlConnection(ConfigurationManager.ConnectionStrings["ProjektConStr"].ConnectionString))
    {
        dbConn.Open();
        using (MySqlCommand cmd = new MySqlCommand("INSERT INTO Submission (CaseId , SubjectId, CenterId, EmployeeName, Reason, Description, Explanation, Date, Done, ChiefLevel) VALUES (MAX(CaseId)+1, @subject_id, @center_id, @employee_name, @reason, @description, @explanation, @date, @done, @chief)", dbConn))
        {
            cmd.Parameters.AddWithValue("date", submission.Date);
            cmd.Parameters.AddWithValue("subject_id", submission.SubjectId);
            cmd.Parameters.AddWithValue("center_id", submission.CenterId);
            cmd.Parameters.AddWithValue("employee_name", submission.EmployeeName);
            cmd.Parameters.AddWithValue("reason", submission.Reason);
            cmd.Parameters.AddWithValue("description", submission.Description);
            cmd.Parameters.AddWithValue("explanation", submission.Explanation);
            cmd.Parameters.AddWithValue("done", false);
            cmd.Parameters.AddWithValue("chief", false);
            cmd.ExecuteNonQuery();
        } 
    }

Answer 1

          Use subquery to select max and insert a value 

INSERT INTO Submission (CaseId , SubjectId, CenterId, EmployeeName, Reason, Description, Explanation, Date, Done, ChiefLevel) VALUES (

          (1 + coalesce((SELECT max(CaseId) FROM Submission), 0))

, @subject_id, @center_id, @employee_name, @reason, @description, @explanation, @date, @done, @chief)