There is no documentation on how to use or reference Assets (Insight) custom fields in the JSM REST API, for example, it's possible to set a value for an Asstets custom field but this is completely undocumented e.g.

curl -D- -u 'admin:sphere' -X POST -H 'Content-Type: application/json' --data '
{
  "serviceDeskId": "1",
  "requestTypeId": "1",
  "requestFieldValues": {
    "summary": "HELP ME",
    "customfield_10123" :[{"key" : "KEY-1"}]
  }
}' '<JIRA BASE URL>/rest/servicedeskapi/request'

One more thing to note here is that it is possible to set the value to "AUTOMATIC-ASSIGN" (instead of the object key "KEY-1") and will trigger the Filter Assign Scope, allowing a dynamic value to be set in the custom field.