To create a rule that contains a static value that can be included in any downloads or outbound webhooks you simply need to navigate to your inbox under Rules
At the top of the screen you can select the option for New Static Value
Once selected the screen will open with a pop-up allowing you to assign the name of the field, as well as the value.
Once you have filled in the fields simply select Create Static Value. The emails once re-parsed will reflect that field and value and can then be used as any other field in a file download, or webhook.