This adds the name/value pair to a signature. The parameter name must consist of an alphabetic character followed by any number of alphanumeric or underscore characters. The parameter value may be any printable string. It will be encoded in UTF8, so it is important that the option charset is used to set the character set properly. If the parameter name is prefixed with an exclamation mark, the notation data will be flagged as critical (see rfc2440:5.2.3.15).