Live UI on the 8.4 Case Manager portal in UI Kit suggests the Section to embed in your Portal to provide the hook into Search is pySearchFieldWorkResponsive . If you are trying to wire in a different search gadget used from an earlier version of Pega then it may not work as the underlying Search architecture is continually evolving. I recommend transitioning to the latest implementation.
If you are looking to leverage a previous rule because you have customized it, I came across the following article yesterday while fielding a similar enquiry. Although the article lands under the Client Lifecycle Management product, all the extension points I checked in it are available from Platform.