For the email type New, the sub-address field in an inbound email action that you can use as described in the Support article Utilizing Email Sub-Addresses. However, if the email type is Forward or Reply, the sub-address field is disabled and cannot be used.
To use the same functionality, the sub-address needs to be defined in the conditions.
Add the following statement in the conditions:
Emails forwarded or replied to this address will trigger this inbound action.