After searching for the same answer and only reading answers pointing to documentation.. which is OK, but not hugely helpful, I decided to post the way I handle this.
If the Reporting parameter is being loaded from a data source then it's very simple to do this using the GUI / report parameter properties.
Just set the Report parameter value to "= AllValues(Fields.XX)"
"Fields.XX" is the value that is set in the "AvailableValues" , "ValueMember" property.
It's that easy.