Encrypt and Decrypt QueryString Parameter Values, using C#

Hi folks, need some help with my code, trying to encrypt 2 parameres, error message from the first prameter LinkID.Text

error message

String string.Trim() (+1 overload(s))

Removes all leading and training white-space characters from the current System.String object.

Error. The left-hand side of an assignment must be a variable, property or indexer.

string returnValue;
            try
            {
                con.Open();
                returnValue = cmd.ExecuteScalar().ToString();
                string link = HttpUtility.UrlEncode(Encrypt(txtLinkID.Text.Trim())) = returnValue;
                string product = HttpUtility.UrlEncode(Encrypt(txtProductID.Text.Trim()));