eHealth Infrastructure (v2020.2) 

{
  "resourceType" : "SearchParameter",
  "id" : "ehealth-communication-search-threadId",
  "text" : {
    "status" : "generated",
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative with Details</b></p><p><b>id</b>: ehealth-communication-search-threadId</p><p><b>url</b>: <a href=\"http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/threadId\">http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/threadId</a></p><p><b>name</b>: threadId</p><p><b>status</b>: active</p><p><b>date</b>: Mar 31, 2020 8:26:16 AM</p><p><b>code</b>: threadId</p><p><b>base</b>: Communication</p><p><b>type</b>: string</p><p><b>description</b>: Search parameter for finding communication based on thread id</p><p><b>expression</b>: Communication.extension('http://ehealth.sundhed.dk/fhir/StructureDefinition/ehealth-thread-id')</p><p><b>xpathUsage</b>: normal</p></div>"
  },
  "url" : "http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/threadId",
  "name" : "threadId",
  "status" : "active",
  "date" : "2020-03-31T08:26:16+00:00",
  "code" : "threadId",
  "base" : [
    "Communication"
  ],
  "type" : "string",
  "description" : "Search parameter for finding communication based on thread id",
  "expression" : "Communication.extension('http://ehealth.sundhed.dk/fhir/StructureDefinition/ehealth-thread-id')",
  "xpathUsage" : "normal"
}