No, XML would not go in a class parameter. As mentioned in the documentation that you link to, the body of the POST should be a JSON object with a “serializedXml” property, whose value is a string of XML. In other words you could POST exactly the example that you copied and pasted.
If you’re asking how you would specifically evaluate a SqlDataSetDefinition the post body would look something like this:
{
"serializedXml": "<org.openmrs.module.reporting.dataset.definition.SqlDataSetDefinition><sqlQuery>select name, description from encounter_type</sqlQuery></org.openmrs.module.reporting.dataset.definition.SqlDataSetDefinition>"
}
Meta: also, please note that while posting to Talk you need to format your code snippets using markdown (by putting 4 spaces in front) which you can automatically do with the </> icon in the text editor. Otherwise the XML is messed up and unreadable.