This can be done by creating a new automation rule under Admin > Workflows > Automations >> Time triggers > New Rule:

 

Conditions:

Match ALL of the conditions

In tickets > if Status > is any of > 'Waiting on 3rd party'

In tickets > Hours since agent responded > is greater than > number of hours (example: 36)


Actions:

Set status as > Closed


Note: 

  1. Please make sure that all mandatory fields for closing a ticket are filled before the status is changed to 'Waiting on 3rd party'.
  2. The 'Hours since' condition for 'custom status' is available from the Pro plan onwards.