Custom parameter of type string. For websites, there are two built-in parameters
URL (name = 'url__') and referrer URL (name = 'ref_url__').
A name must begin with US-ascii letters or underscore or UTF8 code that is greater than 127 and consist of US-ascii letters or digits or underscore or UTF8 code that is greater than 127. This is checked by the regular expression '^[a-zA-Z_-�][a-zA-Z0-9_-�]*$'.