Override Validate Request metho in saveHandler of Repository class.

 private class MySaveHandler : SaveRequestHandler<MyRow>
        {
            private string password;


                protected override void ValidateRequest()
                {
                    base.ValidateRequest();

                    if (IsUpdate)
                    {
                        CheckPublicDemo(Row.UserId);

                        if (Row.IsAssigned(fld.Password) && !Row.Password.IsEmptyOrNull())
                        password = Row.Password = ValidatePassword(Old.Username, Row.Password, false);

                        if (Row.Username != Old.Username)
                            Row.Username = MySaveHandler.ValidateUsername(this.Connection, Row.Username, Old.UserId.Value);

                        if (Row.DisplayName != Old.DisplayName)
                            Row.DisplayName = ValidateDisplayName(this.Connection, Row.DisplayName, Old.UserId.Value);
                    }

                    if (IsCreate)
                    {
                        this.Row.Username = ValidateUsername(this.Connection, this.Row.Username, null);
                        this.Row.DisplayName = ValidateDisplayName(this.Connection, this.Row.DisplayName, null);
                        password = ValidatePassword(Row.Username, Row.Password, true);
                    }
             }

            public static MyRow GetUser(IDbConnection connection, BaseCriteria filter)
            {
                var row = new MyRow();
                if (new SqlQuery().From(row)
                    .Select(
                        fld.UserId,
                        fld.Username,
                        fld.DisplayName,
                        fld.PasswordHash,
                        fld.PasswordSalt,
                        fld.IsActive)
                    .Where(filter)
                    .GetFirst(connection))
                {
                    return row;
                }

                return null;
            }

            protected override void GetEditableFields(HashSet<Field> editable)
            {
                base.GetEditableFields(editable);

                if (!Authorization.HasPermission(Administration.PermissionKeys.Security))
                {
                    editable.Remove(fld.Source);
                    editable.Remove(fld.IsActive);
                }
            }

            private static bool IsInvariantLetter(Char c)
            {
                return (c >= 'A' && c <= 'Z') ||
                    (c >= 'a' && c <= 'z');
            }

            private static bool IsDigit(Char c)
            {
                return (c >= '0' && c <= '9');
            }

            private static bool IsValidEmailChar(Char c)
            {
                return IsInvariantLetter(c) ||
                    IsDigit(c) ||
                    c == '.' ||
                    c == '_' ||
                    c == '@';
            }

            public static bool IsValidUsername(string name)
            {
                if (name == null ||
                    name.Length < 0)
                    return false;

                var c = name[0];
                if (!IsInvariantLetter(c))
                    return false;

                for (var i = 1; i < name.Length - 1; i++)
                {
                    c = name[i];
                    if (!IsValidEmailChar(c))
                        return false;
                }

                return true;
            }

            public static string ValidateUsername(IDbConnection connection, string username, Int32? existingUserId)
            {
                username = username.TrimToNull();

                if (username == null)
                    throw DataValidation.RequiredError(fld.Username.Name, fld.Username.Title);

                if (!IsValidUsername(username))
                    throw new ValidationError("InvalidUsername", "Username",
                        "Usernames should start with letters, only contain letters and numbers!");

                var existing = GetUser(connection,
                    new Criteria(fld.Username) == username |
                    new Criteria(fld.Username) == username.Replace('I', 'İ'));

                if (existing != null && existingUserId != existing.UserId)
                    throw new ValidationError("UniqueViolation", "Username",
                        "A user with same name exists. Please choose another!");

                return username;
            }



            protected override void SetInternalFields()
            {
                base.SetInternalFields();

                if (IsCreate)
                {
                    Row.Source = "site";
                    Row.IsActive = Row.IsActive ?? 1;
                }

                if (IsCreate || !Row.Password.IsEmptyOrNull())
                {
                    string salt = null;
                    Row.PasswordHash = GenerateHash(password, ref salt);
                    Row.PasswordSalt = salt;
                }
            }

            protected override void AfterSave()
            {
                base.AfterSave();

                BatchGenerationUpdater.OnCommit(this.UnitOfWork, fld.GenerationKey);
            }
        }