If you are just after the postcode, you can get it from the HTML using regex:
{"_id":"testrightmove","startUrl":["https://www.rightmove.co.uk/property-for-sale/find.html?locationIdentifier=REGION^257&maxPrice=70000&propertyTypes=detached%2Csemi-detached%2Cterraced&primaryDisplayPropertyType=houses&includeSSTC=false&mustHave=&dontShow=&furnishTypes=&keywords="],"selectors":[{"id":"property","type":"SelectorLink","parentSelectors":["property-card"],"selector":"div.propertyCard-details a.propertyCard-link","multiple":false,"delay":0},{"id":"description","type":"SelectorText","parentSelectors":["property"],"selector":"h1.fs-22","multiple":false,"regex":"","delay":0},{"id":"address","type":"SelectorText","parentSelectors":["property"],"selector":"div.left address.pad-0","multiple":false,"regex":"","delay":0},{"id":"property-card","type":"SelectorElementClick","parentSelectors":["_root"],"selector":".propertyCard-wrapper","multiple":true,"delay":"2500","clickElementSelector":".pagination-direction--next","clickType":"clickMore","discardInitialElements":"do-not-discard","clickElementUniquenessType":"uniqueCSSSelector"},{"id":"Search Criteria","type":"SelectorText","parentSelectors":["_root"],"selector":"h1","multiple":false,"regex":"","delay":0},{"id":"Price","type":"SelectorText","parentSelectors":["property"],"selector":".property-header-price strong","multiple":false,"regex":"","delay":0},{"id":"Added on Rightmove","type":"SelectorText","parentSelectors":["property"],"selector":"div#firstListedDateValue","multiple":false,"regex":"","delay":0},{"id":"Agent","type":"SelectorText","parentSelectors":["property"],"selector":"#aboutBranchLink strong","multiple":false,"regex":"","delay":0},{"id":"Phone Number","type":"SelectorText","parentSelectors":["property"],"selector":".bdr-b .fs-19 strong","multiple":false,"regex":"","delay":0},{"id":"postcode","type":"SelectorHTML","parentSelectors":["property"],"selector":"script:contains('postcode')","multiple":false,"regex":"(?<=postcode\\\":\\\")[^\"]*","delay":0}]}