Update a custom field using email

The help notes within https://www.liquidplanner.com/support/articles/email-integration/ suggests that there is a way to update the custom field with an email subject set to the custom field namevalue, using double quotes to encapsulate the field e.g. "custom status""new value"
When this is done the field will not update. I've tried encapsulating around the entire field/value pair e.g. "custom status*new value" as mentioned in later part of https://www.liquidplanner.com/support/articles/custom-fields/ and that does not work either.
Is this even possible as it would be really useful to be able to use this feature if it is possible?