Unfortunately, this isn’t possible because currently the Item Description widget cannot be interacted with at all via Monday’s API. (So not only is it not available in Monday’s native automations/workflows, but—at least as of today—it’s also not possible to do in another platform using Monday’s API)
The API basically just doesn’t notice or care about the Item Description widget in any way. Even when using the API to get ALL of an item’s fields/values and other metadata, Item Description isn’t in there anywhere.
There’s an existing feature request for this functionality; please go vote!