gistfile1.txt

/**
 * @NApiVersion 2.1
 * @NScriptType Restlet
 */
define([
    "N/log",
    "N/search",
], function (log, search) {
    
    function post(context) {
      log.debug('POST1 Context', context);
        return JSON.stringify(getCustomRecords(context));
    }

    function getCustomRecords(context) {
        var mycustomSearch = search.create({
            type: search.Type.CUSTOM_RECORD + '1589',
            title: 'Search Title',
            id: 'customsearch_my_second_so_search',
            columns: [{
                name: 'id'
            }, {
                name: 'custrecord3987'
            }],
            filters: [{
                name: 'custrecord3987',
                operator: search.Operator.EQUALTO,
                values: [context.custrecord3987]
            }],
  
        });      
      
      return mycustomSearch;
      
    }

    return {
        post: post,
    };
});

python script payload

...
payload = {
 "custrecord3987": "12345678",
}
...

Thank you!