This option can also be set in the Rainmeter.data file. If set there, it will be used as the global setting for all WebParser measures unless overridden in an individual measure(s).
But it does not say how it should be defined. I simply tried to append it to the file
Please note, in future Rainmeter versions greater than 4.5.0, the ".dll" portion of the section name will be dropped for these settings, and it will just be [Webparser].